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LIBRARY 

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V     CALIK)RKIA 


FROM   THE   OPTOHETRIC   LIBRARY 

or      — 

MONROE    JEROME    HIRSCH 


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THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


GIVEN  WITH  LOVE  TO  THE 

OPTOMETRY  LIBRARY 

BY 

MONROE  I.  HIRSCH,  O.D.,  Ph.D. 


THE 

MUSCLES  of  the  RYE 

A  Treatise   on   the    Optical 

Functions  of  the  Muscles  in 

Normal  and  Abnormal  States 

By 

GEORGE  A.  ROGERS 

Illustrated 

Published  by  the  Professional  Press,  Inc..  Chicago.  111. 

C)  6 

OPTOMSTRY 


Copyrighted, 
1922. 


OFTO 


TABLE  OF  CONTENTS 


Chapter  I 7 

Normal  Eyes — Muscle  Functions — Forms  of  Muscles — Classifica- 
tion— Optical  Functions — Nervous  Control — Motor-Nerve  Centres 
— System  of  Control. 

Chapter  II 15 

Pupillary  Control — Dilation  of  the   Pupil — Motor-Nerve   Stimulus 
— Pupillary   Reflex — Tinted   Lenses — Magnified   Pupil — Pupillary 
Discs — Pupillary  Distance. 

Chapter  III 23 

Refraction  Control — Dynamic  Refraction — Ciliary  Action — The 
Ciliary  Body — Motor-Nerve  Control — Sensory  Initiation — Motor- 
Nerve  Ganglia — Amplitude  of  Accommodation — Demand  for  Ac- 
commodation— Presbyopia — Neurometry  of  Accommodation — The 
Time  Element. 

Chapter  IV 41 

Abnormal  Structures — Functional  Effects — Myopia — Inductive  Ef- 
fects— Auxiliary  Effects — Far  and  Near  Points — Presbyopia — 
Hyperopia — Further  Examination — Finding  Hyperopia — Subjective 
Findings  —  Ciliary  Eccentricities  —  Repression — Astigmatism — Hy- 
peropic  Induction. 

Chapter  V 62 

Vision — The  Retina — Visual  Acuity — Foveal  Vision — Fixation — 
Visual  Axis — Indirect  Vision — Cardinal  Points — The  Sensory 
Tracts — Binocular  Single  Vision — Normal  Violation  of  the  Rule — 
Stereoscopic  Vision — Image  Displacements — Visual  Deception. 

Chapter  VI 82 

Axillary  Control — Mountings  and  Muscles — Binocular  Pairs — The 
Medial  Planes — Primary  Position — Functional  Activities — Normal 
Versions — Version  Coordinates — Version  Characteristics — Version 
Tests — Version  Anomalies  —  Homophorias  —  Version  Exercises  — 
Oculo-Didactics — Character  Studies. 


Chapter  VII 100 

Duction  of  the  Eyes — Contest  of  Functions — Normal  Ductions- 
The  Meter  Angle — The  Object  Distance — Convergence  vs.  Adduc- 
tion— Normal  Near  Vision — Fixation  of  Object — Duction  Muscles 
— Nerve  Control — Duction  Tests — Reflex  Influences. 

Chapter  VIII 122 

Muscular  Imbalance — The  Homophorias — The  Heterophorias — Di- 
rection of  Imbalance — Motor-Nerve  Controls — Induction  Influence 
— Intrinsic  Inductions — Extrinsic  Combinations — Extrinsics  with 
Intrinsics — Functional  Neutrality — Vertical  Influences — Near  Vis- 
ion Balance — The  Cyclophorias. 

Chapter  IX  -  -  -  -  -  -  -     140 

Static  Phorometry — The  Target — Device  and  Target — Binocular 
Test — Diagnosis  of  Imbalance — Esophoric  Measurement — Exopho- 
ric  Measurement — Exophoric  Induction — Functional  Equilibrium 
— Vertical  Tests — Muscle  Neutralization — Prism  Measurement^ 
Oblique  Muscle  Tests. 

Chapter  X 165 

Dynamic  Phorometry — Muscle  Tests  for  Near — Addition  for  Near 
— Special  Dynamic  Methods — Prism  and  Cover  Test — Motion  Tests 
— Rotating  Prism  Test— Dynamic  Skiametry  Test — Dioptric  Equiv- 
alents. 

Chapter  XI 185 

Muscle  Exercise — Methods  of  Exercise — The  Natural  Method — 
Artificial  Methods — Trial  Case  Devices — Risley  Rotary  Prisms — 
Rotary  Cylinders — The  Amblyoscope — The  Phorometer — Genothal- 
mic  Refractor — De  Zeng  Phoro-Optometer — The  Woolf  Ski-Op- 
tometer — General  Remarks. 


FOREWORD 


THE  following  treatise  on  the  ocular  muscles  represents  the 
thought  and  work  of  several  years,  concentrated  upon  this  im- 
portant subject,  which  is  now  put  into  usable  shape  and  offered 
by  the  author  to  the  profession  of  optometrists  of  the  country  as  a 
contribution  to  their  rapidly  growing  efficiency  in  the  knowledge  and 
practice  of  their  profession.  It  has  long  been  apparent  to  the  student, 
and  still  more  pressingly  evident  to  the  practitioner  of  optometry 
that  a  working  knowledge  of  refraction  of  the  eye  involves  more 
than  an  acquaintance  with  the  laws  and  phenomena  of  pure  optics. 
He  must  understand  the  dynamic  mechanism  by  which  a  pair  of  eyes 
are  brought  into  conformance  with  these  laws,  and  produce  these 
phenomena.  And,  as  all  the  dynamic  phases  of  vision  are  brought 
about  by  means  of  ocular  muscles,  controlled  through  their  nerve 
supply,  this  is  equivalent  to  saying  that  he  must  have  a  working 
knowledge  of  the  muscles  of  the  eyes  and  their  functioning,  in  both 
normal  and  abnormal  states,  at  least  so  far  as  their  optical  relations 
are  concerned. 

Ocular  muscle  functions,  and  their  anomalies,  admittedly  con- 
stitute one  of  the  most  vexed  and  troublesome  set  of  problems  in  the 
field  of  refraction.  More  confusion  has  gathered  around  them  than, 
around  any  other  subject  in  the  optometrist's  mental  kit.  It  may  be 
added  that  more  foolish  theories  have  been  propounded,  and  more 
unsound  practices  advocated  and  indulged  in,  than  one  cares  to  dwell 
upon.  Yet  the  matter  lends  itself,  like  every  other  physiologic  prob- 
lem, to  clear  and  logical  solution,  if  only  it  be  approached  by  clear  and 
logical  thinking.  It  is  to  the  clearing  away  of  the  confusion,  and  to 
the  straightening  of  the  crooked  paths  which  have  led  into  such  hope- 
less entanglement  that  the  author  has  addressed  himself  in  this  book. 

The  only  way  to  find  the  right  path,  after  having  missed  the 
road  is  to  go  back  to  the  starting  point  and  start  again.  This  is  the 
course  which  the  author  has   pursued.     Every   detail   of   organic 


6  MUSCLES   OF  THE   EYE 

structure  which  has  a  direct  bearing  upon  the  optical  functioning  of 
the  eyes  is  carefully  gone  over.  Step  by  step  the  mechanism  and 
functioning  of  these  structures  are  examined  and  explained.  Sev- 
eral new  and  (as  far  as  the  author  is  aware)  original  explanations 
are  offered  for  hitherto  unexplained  phenomena ;  all  of  which,  how- 
ever, it  is  believed  are  vindicated  and  borne  out  by  the  facts  in  the 
case.  Throughout,  the  practical  aspect  of  the  matter  which  pri- 
marily interests  the  optometrists,  namely,  the  test  whether  the  func- 
tion operates  normally  or  not,  is  made  paramount. 

In  offering  this  fruit  of  his  thought  and  labor  to  the  profession, 
the  author  has  but  one  motive — to  prepare  the  way  of  the  optometrist 
and  to  make  his  paths  straight.  Its  task  will  have  been  fulfilled,  its 
purpose  accomplished,  when  every  practicing  optometrist  shall,  by 
its  aid,  intelligently  explore  and  correct  the  muscle  function  of  every 
patient  as  an  integral  part  of  refraction. 

GEO.  A.  ROGERS. 
May  I,  1922,  Chicago,  111. 


MUSCLES   OF  THE   EYE 


CHAPTER  I. 

Normal  Eyes. 

To  discharge  their  visual  offices  most  efficiently,  the  eyes  must 
have  certain  normal  qualities  and  powers,  some  of  them  of  a  struc- 
tural character,  and  others  functional.     These  normal  qualities  are, 
I.  Structural, 

1.  Normal  transparency  of  the  dioptric  media. 

2.  Normal  static  refraction,  emmetropia. 

3.  Normal  muscular  balance,  orthophoria. 
II.  Functional, 

4.  Normal  action  of  ocular  muscles. 
5.  Normal  acuity  of  vision. 

Corresponding  to  the  above  list  of  normal  qualities,  there  are 
defects  tending  to  impair  the  efficiency  of  the  eyes  as  visual  organs. 
Defects  of  structure  often  disturb  or  disarrange  the  functional  activi- 
ties, but  are  structural  defects  only.  A  functional  defect  is  not  of 
this  kind,  but  an  abnormality  of  action. 

Defects  relating  to  the  first  and  last  qualities  in  the  above  list 
tend  to  impair  vision  but  are  not  correctable  with  lenses.  Hence, 
opacities  of  the  media  and  subnormal  acuity  of  vision  are  not  de- 
fects that  come  within  the  province  of  optometry  or  optometric 
practice,  although  the  optometrist  may  be  able,  in  some  cases,  to 
ameliorate  these  conditions.  He  should  observe  them,  and  take  them 
into  account  in  making  visual  tests,  and  also  in  prescribing  lenses 
for  defects  that  may  be  corrected  by  them. 


8  MUSCLES  OF  THE   EYE 

Muscular  Functions. 

A  muscle  has  but  one  primary  function.  It  can  contract.  The 
relaxation  of  a  muscle  is  its  passive  subsidence,  after  contraction, 
and  is  not  to  be  considered  as  a  function. 

As  a  muscle  is  composed  of  many  thread-like  fibers,  extending 
in  the  same  direction  as  the  muscle  of  which  they  are  a  part,  contrac- 
tion shortens  all  of  these  fibers  and  draws  the  extremities  of  the 
muscle  toward  each  other.  It  therefore  produces  always  a  pulling 
eft'ect.  A  muscle  cannot  be  made  effective  for  pushing,  except  in- 
directly. 

But  a  muscle  is  not  self-contracting.  To  contract  it  requires 
the  stimulus  of  a  motor  nerve;  and  the  stimulus  to  the  muscle 
through  the  motor  nerve  may  be  voluntary  or  involuntary.  Most  of 
the  activities  of  the  ocular  muscles  are  of  the  involuntary  class ;  and 
these  sometimes  have  to  act  in  opposition  to  the  natural  voluntary 
activities. 

What  the  efifects  of  a  muscular  contraction  are  depends  upon 
the  arrangement  of  the  muscular  fibers,  or  the  form  of  the  muscle, 
and  its  attachments  to  other  tissues.  To  produce  an  optical  effect, 
or  to  function  optically,  the  muscle  must  operate  tissues  having 
optical  functions,  for  a  muscle  has  no  intrinsic  optical  quality  or 
function. 

We  must  therefore  look  to  the  tissues  that  an  ocular  muscle 
operates  for  its  optical  efifects  or  functioning. 

Forms  of  Muscles. 

The  ocular  muscles  are  of  three  varieties  of  form,  adapted  to 
the  offices  or  functions  to  which  they  are  assigned.    These  are, 

1.  The  circular  or  sphincter  form. 

2.  The  radial  or  dilator  form. 

3.  The  straight  or  longitudinal  form. 

The  circular  form  is  a  ring-like  arrangement  of  the  fibers  around 
an  aperture  of  some  sort.  Contraction  of  the  muscle  constricts  or 
tends  to  reduce  the  size  of  the  aperture.    Hence  sphincters. 


MUSCLES   OF  THE   EYE  9 

The  radial  form  consists  of  straight  fibers  extending  out  from 
the  margins  of  an  aperture.  Contraction  of  these  fibers  tends  to 
enlarge  or  dilate  the  aperture  from  which  they  radiate.  Hence 
dilators. 

These  two  sets  of  muscles,  sphincters  and  dilators,  when  com- 
bined in  one  tissue,  are  antagonistic  to  each  other,  but  co-operate  in 
controlling  the  size  of  the  aperture. 

The  straight  or  longitudinal  form,  when  not  radially  arranged, 
consists  of  a  single  muscle,  extending  from  one  tissue  to  another, 
and  attached,  at  its  extremities,  to  both  by  tendons.  Usually  one  of 
the  tissues  is  more  firm,  stable  or  immobile  than  the  other,  so  that 
contraction  of  the  muscle  draws  the  less  stable  tissue  toward  the 
more  stable  one  to  which  the  other  extremity  of  the  muscle  is  an- 
chored. This  produces  a  movement,  perhaps  a  rotation,  of  the 
mobile  tissue  in  that  direction.  / 

Classification. 

The  ocular  muscles  are  naturally  divided,  according  to  their 
location,  into  two  main  groups  or  divisions,  the  intrinsic  and  the  ex- 
trinsic muscles.  As  indicated  by  these  terms,  the  intrinsics  lie 
within  the  eye,  and  the  extrinsics  are  outside  of  it,  but  attached  to 
it  at  one  extremity. 

The  intrinsic  muscles  consist  of  two  classes,  operating  different 
but  associated  functions,  and  the  muscles  of  each  of  these  two  classes 
have  the  sphincter-and-radial  arrangement  of  muscular  fibers,  indi- 
cating antagonistic  co-operation  or  co-ordination  in  exercising  con- 
trol. The  extrinsic  muscles  are  so  placed  and  attached  that  their 
contraction  rotates  the  eye  in  its  orbit,  either  singly  or  both  eyes  at 
once,  and  in  the  same  or  in  opposite  directions. 

Optical  Functions. 

Tiie  optical  functions  of  the  ocular  muscles,  intrinsic  and  ex- 
trinsic, are  as  follows : 
I.  Intrinsic, 

1.  The  iris  muscles  control  the  size  of  the  pupillary  aper- 
ture for  the  admission  of  Hght  into  the  eye. 

2.  The  ciliary  muscles  control  the  increase  and  decrease 
of  convexity  of  the  crystalline  lens. 


10  MUSCLES  OF  THE   EYE 

II.  Extrinsic, 

3.  Muscles  that  control  the  direction  of  the  visual  axis  of 
each  eye,  and  the  relative  direction  of  both  axes. 

Although  a  muscle  has  but  the  one  primary  function,  by  its 
contraction  it  moves  tissues  into  different  relative  positions,  and 
these  are  efifective  as  optical  functions  of  the  eyes. 

Co-operative  Antagonism. 

In  all  of  the  "settings"  of  the  ocular  muscles  the  principle  of 
co-operative  antagonism,  or  antagonistic  co-operation,  is  observed. 


FIGURE  1. 

Different  arrangements  of  (M)  muscular  fibers  in  ocular  muscles  for  differ- 
ent functions:  1,  (S)  Sphincter  and  (R)  Radial  arrangement:  2,  Fan-shaped 
arrangement  (A,  anchor;  M.  muscles) ;  3,  Longitudinal,  Meridianal  or  Lineal 
arrangement   (T,   tendons;    M,   muscles,   A,  anchor). 

Control  requires  that  a  function  should  operate  in  either  direction, 
or  in  opposite  directions,  so  that  one  may  check  the  other,  or  either 
may  undo  what  the  other  has  done,  leaving  nothing  to  natural 
forces,  such  as  the  force  of  gravity,  elasticity,  etc.  This  arrange- 
ment alone  gives  complete  muscular  control  of  a  function. 

As  a  single  muscle  can  pull,  but  can't  push,  the  antagonistic 
pairing  of  opposing  muscles  is  necessary  to  provide  control.  In  the 
iris  this  arrangement  is  fully  provided  for  and  recognized.  In  the 
ciliary  there  is  a  similar  arrangement  of  muscular  antagonists,  but 
the  full  details  of  their  antagonistic  co-operation  are  wanting,  though 
the  need  of  it  is  apparent,  and  like  the  iris  may  soon  be  discovered. 
It  is  optically,  and  therefore  physiologically  certain. 

The  extrinsic  muscles  of  each  eye  are  in  antagonistic  pairs, 
rotating  the  eye  in  opposite  directions.  Binocularly,  the  two  eyes 
may  be  rotated  together  in  the  same  direction,  and  this  is  a  natural 


MUSCLES   OF  THE   EYE  11 

and  voluntary  movement,  employing  co-ordinate  pairs  of  muscles. 
But  they  may  also  be  rotated,  by  the  same  muscles,  but  in  different 
pairs,  in  opposite  directions,  and  the  last  are  antagonistic  to  the  first. 

Nervous  Control. 

But  back  of  the  muscle  there  is  a  motor  nerve,  and  back  of  this 
there  is  a  ganglionic  nerve-center,  endowed  with  the  power  to  re- 
ceive and  interpret  sensory  demands  that  are  brought  to  it  by  sensory 
nerves,  and  to  generate  and  transmit  motor-nerve  stimulus  to  the 
muscles  that  are  required  to  act  to  allay  a  sensory  irritation.  The 
ganglionic  center  may  be  the  brain  itself,  or  some  outlying  station, 
delegated  to  exercise  control  over  a  particular  function. 

Brain  directed  motor-nerve  force,  which  is  exercised  or  not,  at 
will,  is  voluntary.  But  the  powers  exercised  by  an  outlying  gangli- 
onic nerve-center  and  confined  to  a  specific  function,  are  involuntary. 
An  involuntary  action  of  this  kind  is  termed  a  "reflex  action"  since 
the  sensory  message,  before  it  can  reach  the  brain,  is  intercepted  and 
reflected  back  as  motor- nerve  stimulus  to  the  muscle  required  to  act. 
It  not  only  relieves  the  brain  power  of  responsibility,  but  discharges 
it  with  a  promptness  and  fidehty  that  the  mind  faculties  would  be 
unable  to  exercise,  for  it  feels  exactly  what  is  wanted,  and  supplies  it. 

The  intrinsic  muscles  of  the  eye,  both  those  of  the  iris  and  the 
ciliary,  are  exercised  by  reflex  action.  Binocularly,  the  co-ordinate 
movements  of  the  eyes,  and  the  exercise  of  the  muscles  in  co-ordinate 
pairs,  to  rotate  the  eyes  in  the  same  direction,  is  a  voluntary  action. 
But  the  exercise  of  the  same  muscles  in  antagonistic  pairs,  to  rotate 
the  eyes  in  opposite  directions,  is  an  involuntary  or  reflex  action. 

Motor-Nerve  Centers. 

The  location  of  the  ganglionic  nerve  centers  that  exercise  in- 
voluntary control  over  muscles,  is  like  the  locating  of  stars  and 
constellations  in  the  firmament.  Much  may  be  known,  but  there  are 
still  undiscovered  activities  to  account  for.  One  of  these  may  be  the 
action  of  the  ciliary  muscles  to  control  the  convexity  of  the  crystal- 
line lens. 

Optometrists  are  not  anatomists  nor  surgeons.  Their  observa- 
tions are  in  optical  effects,  and  by  them  they  judge  the  function  and 


12 


MUSCLES   OF   THE   EYE 


the  potentiality  of  a  muscle  to  discharge  that  function  efficiently. 
If  it  cannot,  it  is  either  because  the  load  is  too  heavy  or  the  control 
is  too  weak.  They  take  off  the  load,  when  it  is  over  loaded,  by 
lenses,  or  strengthen  the  function  by  enforced  exercise,  if  it  is  ab- 
normally weak.  For  either  of  these  corrective  measures,  optom- 
etrists employ  lenses  only,  but  they  are  the  best,  and  often  the  only 
corrective  agents. 


....e 


FiaURE  2. 

Elements  of  Reflex  Arch:  O,  objective  point  of  contact  and  irritation; 
S,  sensory  nerve  from  O  to  G;  G,  ganglionic  nerve-center,  direct  source  of 
stimulation;  N,  motor  nerve  from  G  to  M;  M,  muscle  stimulatetl  by  N;  T,  trunk 
line  of  general  nervous  system. 

System  of  Control. 

In  the  operation  of  the  muscular  functions  of  the  eyes  by  reflex 
action,  the  system  of  control  is  not  unlike  that  of  the  operation  of  a 
continental  telegraph  or  telephone  system,  in  which  a  central  office 
exercises  supreme  command,  but  delegates  the  minor  details  to  out- 
lying stations  or  exchanges.  In  some  respects  these  exchanges  re- 
semble the  "tower  man"  of  a  great  railway,  operating  the  switches 
under  the  direct  supervision  of  the  "train  dispatcher''  but  function- 
ing individually  for  local  traffic  over  the  line. 

Nor  is  the  modern  "wireless"  means  of  communication  left  out 
of  the  scheme,  for  light  arrives  at  its  destination  on  no  material 
wires,  but  wings  its  ways  through  ether  to  the  air,  and  through  the 
air  to  a  sensitive  instrument  that  records  what  it  has  to  say  of  the 
world  around,  so  that  the  messages  from  objects  to  the  eye  are 
registered  upon  the  sensitive  field  of  the  retina,  from  which  point 


MUSCLES   OF   THE   EYE  13 

they  proceed  by  material  conveyors,  the  sensory  nerves,  to  the  gangli- 
onic motor-nerve  centers,  where  their  messages  are  received  and 
interpreted,  and  from  which  the  necessary  stimulus  for  muscular 
action  is  sent  out. 

The  eye  is  the  instrument  that  transcribes  the  messages  brought 
to  it  by  wireless,  which  is  in  the  universal  language  of  physical  sci- 
ence, that  the  brain  physiologically  reads  and  interprets  into  action. 
Does  it  seem  as  though  the  exercise  of  these  physiologic,  neuro- 
logic and  psychologic  functions  or  powers  were  "copied"  after  the 
telegraph,  telephone  or  railway  systems ;  or  that  the  latter  are  weak 
imitations  of  the  former  powers  that  have  been  exercised  by  man 
since  his  creation?  And  at  the  present  time,  which  is  the  most  per- 
fect in  its  operation?  And  whence  comes  the  neuro-sensory  and 
neuro-motive  force  that  operates  the  human  system  but  from  dyna- 
mos that  are  embraced  within  and  concealed  by  its  various  tissues,  as 
the  operation  of  the  eye  is  ? 

The  only  apparent  superiority  of  the  materially  constructed 
artificial  system  is  its  perpetuity,  which  is  limited  of  course,  but  may 
be  renewed,  repaired,  undergo  substitution  of  parts,  but  must  be 
constantly  charged  and  recharged  with  the  electric  current,  provided 
by  the  human  machine,  to  keep  it  in  operation.  Without  the  latter 
it  dies  even  more  certainly  and  quickly ;  and  without  man  to  observe 
its  functioning,  as  well  as  to  direct  its  operation,  of  what  conse- 
quence would  it  all  be  ? 

So,  it  would  seem,  that  whatever  credit  we  may  give  ourselves, 
for  scientific  discovery  and  invention,  nature  has  anticipated  us  by 
some  millions  of  years,  and  provided  a  model  for  us  to  follow,  a 
"master  machine"  for  us  to  copy.  We  will  find  this  to  be  the  case 
with  the  lesser  details  of  the  human  mechanism,  as  the  dioptric 
media  of  the  eye,  constructed  in  the  lens  form.  Their  transparency 
exceeds  that  of  the  finest  optical  material  or  glass  that  we  can  pro- 
duce. And,  without  the  eye  as  the  final  optical  mechanism,  what 
purpose  would  all  of  our  lenses  serve?  We  have  no  natural  bi-focal, 
but  the  power  of  accommodation,  while  active,  makes  it  unnecessary. 

As  to  the  principle  of  the  bifocal  lens,  or  rather  of  the  wonder- 
ful Kryptok  variety  of  it,  the  model  for  that  is  in  the  eye,  for  the 


14  MUSCLES   OF  THE   EYE 

crystalline  lens  is  countersunk  between  media  of  less  optical  density 
and  has  the  excessive  curvature  required  to  give  it  a  special  lens 
action  in  addition  to  the  other  lens  elements  that  enclose  it.  But  it 
has,  by  the  exercise  of  accommodation,  the  function  of  increasing 
that  curvature  and  power,  which  no  Kryptok  possesses.  In  the  eye 
you  have  modern  optical  and  optometric  science,  as  it  were,  in  a  nut- 
shell. 


MUSCLES   OF   THE   EYE  IS 

CHAPTER  II. 

Pupillary  Control. 

This  is  a  function  that  is  exercised  directly  by  the  muscles  of 
the  iris,  an  opaque  pigmented  or  colored  screen  directly  back  of  the 
cornea,  from  whose  posterior  surface  it  is  separated  by  a  space  of 
about  2>2  millimeters. 

It  lies,  or  is  suspended,  just  forward  of  the  crystalline  lens, 
so  that  its  inner  margins,  surrounding  a  central  round  aperture,  the 
pupil,  rest  against  the  anterior  capsule  of  the  lens.  The  transpar- 
ent aqueous  humor  fills  the  space  between  it  and  the  cornea,  and 
also  the  space  that  remains  between  it  and  the  outer  margins  of  the 
lens  and  the  suspensory  ligament.  It  is  an  appendage  of  the  ciliary 
body,  or  of  the  choroid  tunic,  of  which  the  ciliary  body  is  a  feature. 

Its  most  important  feature,  besides  its  opacity  and  color,  is  the 
round  aperture  in  its  center,  the  pupil,  through  which  light  is  ad- 
mitted into  the  inner  eye.  Normally  of  about  4  mm.  in  diameter, 
the  size  of  the  pupil  may  be  reduced  or  increased  by  the  means  of 
muscles,  arranged  in  the  sphincter-and-radial  form.  The  sphincter 
muscles  are  located  in  the  margin  of  the  iris  surrounding  the  pupil. 
Their  contraction  constricts  or  reduces  the  size  of  the  pupil ;  while 
the  radial  muscles  extend  from  the  sphincter  to  the  outer  borders  of 
the  iris,  and  their  contraction  enlarges  or  dilates  the  pupil. 

The  antagonistic  arrangement  of  these  muscles  indicates  their 
opposition  to  each  other,  and  yet  they  co-operate  harmoniously  for 
the  control  of  the  size  of  the  pupillary  aperture.  When  the  sphinc- 
ters contract  the  radials  relax,  and  vice  versa,  so  that  which  of  these 
actions  is  taken  depends  upon  the  stimulus  that  is  sent  to  one  or  the 
other,  and  that  in  turn  depends  upon  the  sensory  demand  for  more 
or  less  light.  The  iris  also  screens  the  margins  of  lens  and  cornea 
from  participation  in  the  lens  action  required  to  focus  incident  or 
neutralize  emergent  light. 

In  case  incident  light  is  not  focused  at  the  retina,  but  is  spread 
in  diffusion  circles  over  it,  contraction  of  the  pupil  reduces  the 
size  and  spread  of  the  diffusion  circles,  and  lessens  the  blur  of 
images,  so  that,  in  this  respect,  its  office  is  that  of  a  diaphragm  to 


16 


MUSCLES   OF   THE   EYE 


sharpen  the  definition  of  images  that  are  blurred  by  diffusion.  The 
pin-hole  disc  excmpHfies  this  function,  and  carries  it  farther  mechan- 
ically. 

Dilation  of  Pupil. 

The  pupil  is  the  gate-way  of  incident  light  from  objects  we  see 
to  the  retina,  and  also  for  special  illumination  of  the  retina  for  the 
purpose  of  making  an  ophthalmoscopic  examination  of  the  fundus 
or  eye-ground,  and  for  skiametry  or  shadow-testing  the  eye.  The 
latter  use  of  it  is  confined  to  those  whose  practice  as  optometrists 
or  oculists  makes  such  examinations  necessary.  In  either  case  the 
illumination  is  too  brilliant  for  visual  purposes. 

But  the  pupil  is  also  the  gate-way  for  the  light  that  emerges 
from  the  eye  in  an  ophthalmoscopic  or  skiametric  examination  of 
the  eye.  Illumination  of  the  retina  for  an  ophthalmoscopic  examina- 
tion is  made  both  wide  and  brilliant,  so  as  to  make  the  details  of 
the  fundus  visible  over  a  considerable  field,  at  least  considerably 
larger  than  the  optic  nervehead  or  disc,  and  an  enlarged  pupil  is 
desirable,  especially  for  a  wide  illumination  of  the  fundus,  and 
advantageous  in  affording  a  better  view  of  it.  But  in  a  majority 
of  cases  optometrists  have  no  trouble  to  make  a  sufficient  examina- 
tion of  it  without  artificial  dilation. 


^-^^^-^-^-N^^f 


12  3 

FIGURE   3. 
Dilatation  anri  constriction  nf  pupil  by  different  intensities  of  light:    1.  for- 
mal pupil   (4   mm.);   2,   constricted   pupil,   due  to   intense  light;    3,   dilated   pupil, 
due  to  dim  light. 

In  making  a  shadow-test  of  the  eye,  the  illumination  does  not 
need  to  be  very  brilliant,  and  as  but  a  very  small  spot  on  the  retina 
is  illuminated,  a  small  pupil,  or  a  normal  one,  is  sufficient.  A  more 
conspicuous  reflex  is  obtained  by  an  enlarged  pupil,  but  as  this  is 
a  test  of  the  refraction  of  the  eye,  an  enlarged  pupil  exposes  areas 
of  the  lens  and  cornea  that  the  iris  shuts  out  for  visual  purposes. 
If  it  is  advantageous  to  have  these  areas  covered  for  visual  pur- 


MUSCLES   OF  THE   EYE  17 

poses,  it  is  of  the  satnc  advantage  to  have  them  exckided  in  any  test 
of  the  refraction.  A  greater  difficulty  to  shadow-testing  is  the 
deep  coloring  of  the  fundus  by  the  choroidal  pigment. 

For  refractive  purposes,  both  subjective  and  objective,  the 
optometrists  may  regard  themselves  as  having  the  advantage  over 
those  who  consider  that  the  pupil  needs  to  be  greatly  dilated  for  the 
purpose  of  making  an  objective  examination  of  or  through  the  diop- 
tric media.  Objective  examination  with  the  ophthalmometer  is 
not  of  this  class,  as  this  is  an  examination  of  the  front  surface  of 
the  cornea  only,  and  of  its  curvature  and  relative  curvature  in  dif- 
ferent meridians,  for  the  detection  of  corneal  astigmatism. 

The  shadow-test  examination  is  one  for  the  measurement  of 
refraction.  As  the  volume  of  light  required  for  such  examination 
is  little,  a  large  or  brilliant  light  defeats  it  by  stimulating  the  accomo- 
modation,  and  to  that  extent  nullifying  the  findings.  It  also  tends 
to  interfere  with  a  later  subjective  examination  by  reducing  the 
reaction  of  the  retina  to  normal  illumination  for  visual  purposes. 
That  is,  the  visual  purple,  or  chemical  agent  that  sensitizes  the  rods 
and  cones,  is  exhausted,  or  its  reaction  to  light  is  reduced,  affecting 
acuity  of  vision. 

Motor-Nerve  Stimulus. 

The  motor  ner^-es  that  supply  the  sphincter  muscles  of  the  iris 
are  said  to  be  tendrils  of  the  third  cranial  nerves,  while  those  that 
supply  the  radials  are  derived  from  the  sympathetic  system.  This, 
as  well  as  the  location  of  the  ganglionic  center  that  supplies  the 
stimulus  are  not  matters  of  any  consequence  to  the  optometrist.  If 
the  function  operates  normally  and  the  pupil  responds  to  light,  the 
function  and  all  that  goes  with  it  may  be  presumed  to  be  there. 

Nor  is  it  to  be  considered  a  static  abnormality  if  the  pupils 
should  appear  to  be  unusually  large  or  small,  nor  even  a  functional 
abnormality  if  they  fail  to  normally  respond  to  light.  A  patient  of 
this  class  may  have  visited  someone  else  before  she  came  to  you ; 
or  she  may  be  under  treatment  with  some  of  the  powerful  alkaloids 
so  generally  used  in  medical  practice,  for  some  ailment  not  con- 
nected with  the  eyes  or  vision,  or  with  ophthalmic  salve,  containing 
atropine  or  cocaine  or  both,  for  infected  eye-lids. 


18 


MUSCLES  OF  THE  EYE 


The  muscles  of  the  iris,  or  the  motor-nerves  that  stimulate 
them,  are  parts  of  a  system  that  extends  to  other  muscles,  exercising 
corresponding  functions.  Hence,  slight  muscular  and  nervous  activ- 
ity of  such  other  function  may  produce  responsive  effects  at  the 
iris  that  its  own  normal  function  does  not  account  for.  These  other 
possible  influences  must  first  be  quieted  before  one  can  judge  the 
source  of  them.  If  the  other  disturbing  factor  is  functional  to  the 
eyes,  that  may  be  reached ;  if  not,  we  cannot  reach  it. 

With  pathological  causes  for  an  abnormal  pupil,  or  surgical 
distortion  or  abnormality,  optometry  is  not  qualified  to  deal,  at  least 
correctively.  As  there  are,  relatively,  very  few  of  them,  one  only 
invites  trouble  to  attempt  more  than  lies  within  his  field.  Superiority 
of  technique  in  that  field  ought  to  satisfy  him. 


FIGURE  4. 

Pupillary  Devices:     1,   Pupillary   disc   or  diaphragm;     2,    pin-hole   disc; 
stenopaic  disc  or  slit;  4,  iris  diaphragm. 


Pupillary  Reflex. 

Optometry  has  no  special  means  of  controlling  the  pupil  by 
artificial  agents,  such  as  lenses,  although  the  latter  slightly  increase 
or  decrease  the  volume  of  light  that  is  directed  into  the  eye ;  by  their 
artificial  control  of  other  muscular  functions  of  the  eye  they  influ- 
ence the  muscles  of  the  iris,  and  thereby  affect  the  pupil. 


MUSCLES   OF  THE   EYE  19 

Nature  or  physiology  has  provided  the  eye  with  the  most  per- 
fect means  of  protecting  itself  from  too  great  a  volume  or  too  high 
brilliancy  of  light  in  the  reflex  control  of  the  pupil.  The  pupil  is 
the  watchdog  against  irritation  that  excessive  light  to  the  eye  pro- 
duces. It  is  always  on  guard  to  save  the  eye  from  any  annoyance  of 
this  kind.  No  carelessness  or  heedlessness  on  our  part,  except  that 
of  voluntarily  going  where  the  light  is  too  bright,  will  deprive  us  of 
this  protection,  as  it  is  an  involuntary  action,  although  nerve-center, 
motor-nerve  stimulation  and  muscular  contraction  are  necessary  to 
effect  it. 

But  we  are  not  wanting  in  voluntary  means  of  protecting  the 
eyes  from  excessive  light.  We  can  shut  them,  or  close  the  opaque 
lids  over  them,  which  is  partly  a  reflex  action  and  partly  a  voluntary 
one.  We  may  also  turn  the  head  away,  or  turn  our  back  to  the 
light.  We  may  also  hold  or  wear  an  opaque  screen  in  such  position 
that  it  shuts  off  light  from  the  eye  but  throws  it  upon  whatever  we 
wish  to  see.  We  can  also  move  away  from  the  source  of  light,  and  to 
darker  places.  But  further,  we  may,  if  the  light  is  artificial,  ah. 
put  it  out  or  turn  it  down. 

These  are  voluntary  acts  that  we  must  deliberate  upon  before 
we  act  in  order  to  determine  what  to  do.  But  before  we  can  con- 
sider the  question  and  decide  it,  the  pupil  acts  involuntarily  while 
we  are  making  up  our  mind.    Such  is  the  nature  of  a  reflex  action. 

Tinted  Lenses. 

Although  lenses  do  not  directly  control  the  action  of  the  pupil, 
the  eye  may  be  protected  from  glare  and  heat  by  smoked  or  tinted 
lenses,  which  obstruct  partially  the  light  that  passes  through  the 
pupil,  but  allow  sufficient  to  pass  for  visual  purposes.  This  tends 
to  tone  down  the  brilliancy  of  light,  so  that  the  irritation  caused  by 
a  too  brilliant  light  is  reduced  and  the  pupillary  action  is  afforded 
that  much  relief. 

Amber  tinted  lenses  are  regarded  as  affording  protection  from 
the  irritating  effects  of  the  reflection  of  light  from  wide  areas  of 
snow  covered  ground,  or  snow-blindness.  The  Crookes  glass  lenses, 
though  designed  to  shut  out  certain  qualities  of  light,  light  of  such 


20  MUSCLES   OF   THE   EYE 

high  wave-frequency  as  to  have  specially  irritating  sensory  effects 
at  the  retina,  are  not  designed  primarily  to  lessen  the  volume  of  light 
that  is  admitted  into  the  eye  because  of  semi-opacity,  or  opacity  of 
any  degree,  except  to  the  ultra-violet  light. 

As  to  the  cause  of  pupillary  constriction,  eyes  vary  greatly  in 
sensitiveness  to  light.  Eyes  that  are  supersensitive  to  its  influ- 
ence have  small  pupils,  and  this  quality  is  termed  "photophobia." 
It  is  impossible  to  shadow-test  such  an  eye,  or  even  to  make  an 
ophthalmoscopic  examination  of  it.  It  is  exemplified  in  the  nocturnal 
animals,  whose  pupils  in  daylight  are  very  small,  but  which  at  night, 
when  prowling  for  their  prey,  become  large.  Such  eyes  are  some- 
times found  in  human  beings,  though  less  exaggerated  than  in  ani- 
mals of  this  variety. 

There  is  a  special  science  or  practice,  professing  to  diagnose 
disease  by  the  peculiarities  of  markings  of  different  areas  of  the  iris. 
But  this  is  not  pupillary,  nor  related  to  its  size. 


FIGURE   5. 
Measurement  of  pupillary  width  or  distances,  inner  edge  of  right  to  outer 
edge  ot  left  iris,  or  vice  versa. 

Magnified  Pupil. 

The  iris  and  pupil  are  directly  back  of  the  cornea  and  aqueous 
humor,  both  of  which  are  in  the  positive  lens  form  and  therefore  have 
a  slight  magnifying  effect.  This  effect  is  increased  if  the  depth  of 
the  pupil,  or  its  distance  back  of  the  cornea,  is  greater  than  normal, 
as  it  is  supposed  to  be  in  myopic  eyes.  This  may  account,  in  part, 
for  an  appearance  of  unusual  size  in  the  pupils  of  myopes,  but  all 
pupils  are  slightly  enlarged  in  appearance  by  such  magnification. 

By  placing  a  positive  lens  before  it,  this  magnification  may  be 
increased,  and  is  at  its  maximum  when  the  lens  is  at  its  focal  dis- 
tance from  the  iris,  or  from  the  virtual  image  of  the  iris  that  the 
corneal  and  aqueous  refraction  produces.    This  means  of  making  a 


MUSCLES   OF   THE    EYE  21 

closer  inspection  of  the  anterior  capsule  of  the  lens,  or  of  the  lens 
itself,  is  employed  for  the  discovery  of  incipient  signs  of  cataract. 
Rut  it  is  also  used  to  concentrate  a  side-light  upon  the  pupil,  or  of 
that  part  of  the  crystalline  lens  that  protrudes  into  it,  for  the  same 
purpose. 

Pupillary  Discs. 

This  is  an  opaque  diaphragm,  with  a  central  aperture,  mounted 
in  a  trial  lens  rim,  so  as  to  facilitate  placing  it  before  the  eye  in  a 
trial- frame  cell.  It  is  a  convenient  device  to  be  employed  when  a 
surgical  operation  has  left  the  pupil  irregular  in  form,  as  it  is  a 
central  round  aperture,  and  a  trifle  larger  than  the  normal  pupil  on 
account  of  its  position  in  the  cell  at  some  distance  from  the  eye. 
Oculists,  after  dilating  the  pupil,  sometimes  employ  it  to  confine  re- 
fraction in  shadow-testing  the  eye  to  normal  visual  areas. 

The  pin-hole  disc  has  a  very  small  aperture  in  the  center.  Its 
use  is  principally  for  the  purpose  of  limiting  diffusion  circles  at  the 
retina  when  the  eye  does  not  focaHze  correctly.  If  vision  is  im- 
proved by  this  means,  it  indicates  that  there  is  an  optical  defect  that 
a  lens  of  the  right  kind  and  power  will  correct  even  better.  It  is  not 
to  be  assumed,  however,  that  if  the  pin-hole  test  does  not  improve 
vision,  there  is  no  such  optical  defect.  The  stenopaic  disc  is  a  modi- 
fied form  of  the  pin-hole  disc,  as  it  is  a  pin-hole  elongated  in  one  di- 
rection. It  exposes  but  one  narrow  slit  to  the  admission  of  light,  and 
this  may  be  made  to  cover  any  desired  meridian  of  the  eye  by  rotation 
in  the  cell. 

Pupillary  Distance. 

This  is  a  binocular  term,  referring  to  the  straight  distance  that 
separates  the  centers  of  the  two  pupils  when  the  visual  axes  are 
parallel;  or  if  the  eyes  are  converged  to  a  near  object,  the  shorter 
distance  that  separates  them,  depending  upon  the  distance  of  the  ob- 
ject from  the  eyes,  but  usually  the  reading  distance  of  ys  meter,  or 
practically  13  inches.  As  there  is  no  distinct  demarcation  of  the  cen- 
ter of  the  pupil,  corresponding  edges  of  the  iris,  right  or  left,  are 
made  the  basis  for  measurement.  These  are  referred  to  as  the  dis- 
tance p.  d.  or  the  near  p.  d. 


22  MUSCLES   OF   THE    EYE 

There  is  a  considerable  variation  between  the  p.  d.  of  different 
persons.  The  average  is  about  2}i  inches,  or  practically  60  mm,, 
counting  an  inch  equal  to  254  mm.  The  pupillary  distance  has 
nothing  to  do  with  the  size  of  the  pupil,  although  it  is  a  very  impor- 
tant mechanical  factor  in  fitting  frames,  or  in  distancing  lenses  from 
each  other.  It  is  mentioned  here  merely  because  it  pertains  to  the 
pupil  which  we  are  discussing  in  this  section. 


MUSCLES   OF  THE   EYE  23 

CHAPTER  III. 

Refraction  Control. 

The  static  refraction  of  an  eye  is  its  lens-power  when  in  a  state  of 
muscular  relaxation  or  rest.  Under  standard  data  as  to  curvature 
and  index,  this  is  estimated  to  be  approximately  +58.50  D.,  which 
is  its  refraction  in  relation  to  air,  and  takes  into  account  the  effects 
of  the  separation  of  the  dioptric  surfaces,  which  are  located  in  the 
anterior  Yz  of  its  axial  depth.  This  gives  the  eye  a  focal-length  of 
approximately   17  millimeters. 

An  eye  may  have  greater  or  less  dioptric  power  than  the  above 
and  be  normal  or  emmetropic,  or  have  the  above  power  without  be- 
ing emmetropic,  for  emmetropia  is  that  static  refraction,  relative  to 
its  axial  depth,  by  which  light  from  distant  points  is  focused  at  the 
retina,  making  clear  images  of  distant  objects  there.  Hence,  an  eye, 
whatever  its  refractive  power,  must  have  an  axial  depth  to  corre- 
spond to  it  to  be  emmetropic.  Its  focal-length  varies  in  the  same 
manner. 

Dynamic  Refraction. 

The  dynamic  refraction  of  an  eye  is  the  lens-power  that  may 
be  added  to  its  static  refraction,  and  withdrawn  from  it  again,  by 
muscular  action.  It  is  effected  by  increasing  the  convexity  of  the 
crystalline  lens  only,  and  principally  at  its  anterior  surface.  This 
increase  of  its  convexity  is  withdrawn  again  by  a  reversal  of  the 
muscular  action  that  produces  it ;  but  is,  we  believe,  due  to  a  muscular 
action  in  either  direction,  which  reverses  the  tension  that  is  applied 
to  the  lens  for  either  purpose. 

The  static  refraction  of  the  crystalHne  lens  is  about  -4-23.50  D. 
This  is  relative  to  (but  not  in)  air,  and  takes  account  of  the  thick- 
ness of  the  lens,  or  the  separation  of  its  surfaces.  It  is  based  on 
an  index  of  1.425  for  the  crystalline  lens  and  of  1.335  for  the  humors 
that  surround  and  enclose  it.  The  index  factor,  relative  to  air,  is 
therefore,  the  difference  between  them,  or  .09.  Hence,  to  accom- 
modate I  D,  there  must  be  a  change  of  convexity  of  i/.09=ii  meter 
curves.  For  glass  of  an  index  of  1.5  in  air,  a  change  of  2  meter 
curves  makes  i  D.  change  in  the  value  of  a  lens. 


24 


MUSCLES  OF  THE   EYE 


The  muscular  force  by  which  these  changes  of  convexity  are 
made  in  the  crystalline  lens  is  applied  directly  at  some  distance  from 
it,  and  a  rather  complex  system  of  transmission  is  required  to  bring 
it  to  bear  upon  the  lens.    This  mechanism  embraces  the  following : 

1.  The  Ciliary  Muscles. 

2.  The  Ciliary  Body  and  Processes. 

3.  The  Suspensory  Ligament. 

4.  The  Capsule  of  the  Lens. 

5.  The  Crystalline  Lens. 

The  manner  in  which  muscular  force  causes  the  lens  to  change 
in  convexity  is  not  entirely  agreed  upon,  but  there  is  no  (luestion 
about  its  being  through  the  above  agents. 

Ciliary  Action. 

It  is  agreed  that  contraction  of  the  ciliary  muscles,  one  or  both 
sets  of  fibers,  first  affects  the  position  or  inclination  of  the  ciliary 
body  and  its  processes.  The  latter,  into  which  the  suspensory  liga- 
ment is  "snubbed,"  changes  the  tension  on  the  ligament,  and  that 


FIGURE   6. 
Ciliary  Rei^ion    of   Eye:     L,    crystalline    lens:    C    and    C.    anterior   and    pos- 
terior lens  capsule:  S,  suspensory  ligament;  B,  ciliary  body;  P,  ciliary  processes; 
M,  M'  and  M",  ciliary  muscles;  I,  iris;  K,  cornea;   G,  pectonate  ligament. 

is  communicated  or  transmitted  to  the  capsule  of  the  lens,  either  an- 
teriorally  or  posteriorally  or  both,  for  the  ligament,  at  the  lens,  di- 


MUSCLES   OF  THE   EYE  25 

vides  into  two  sections,  one  being  attached  anteriorally  and  the  other 
posteriorally  to  the  capsule.  It  is  the  tension,  or  release  of  it,  thus 
transmitted  to  the  lens  that  gives  it  greater  or  less  convexity. 

Of  the  two  prevailing  theories,  according  to  one  of  them  the 
lens  is  normally  under  tension ;  and  action  of  the  ciliary  releases 
it,  which  allows  the  lens,  of  its  own  elasticity,  to  convex  itself; 
while  relaxation  of  the  muscles  causes  the  tension  to  be  resumed, 
thus  bringing  the  lens  back  to  normal  static  convexity ;  on  the  other 
hand,  the  second  theory  holds  that  contraction  of  the  ciliary  muscles 
forces  increased  convexity  into  the  lens,  and  their  relaxation  allows 
it  to  subside  back  to  normal  form.  Neither  theory  is  entirely  satis- 
factory, for  either  makes  the  elasticity  of  the  lens  the  essential 
factor,  either  for  increasing  or  reducing  the  convexity. 

According  to  the  first  theory  also,  increased  convexity  is  over 
the  entire  anterior  surface  of  the  lens ;  but  according  to  the  second 
a  small  lenticular  bulge  is  raised  on  the  anterior  surface,  just  back 
of  the  pupil,  while  its  peripheral  areas  are  really  flattened.  The  len- 
ticular bulge  is  accounted  for  by  the  resistance  of  the  harder  nucleus 
of  the  lens  to  the  pressure  that  is  brought  to  bear  upon  it,  so  that  high 
increase  of  convexity  is  given  to  a  small  area  at  the  exact  point  where 
it  is  required,  the  pupil.  The  first  is  the  so-called  Helmholtz  theory, 
the  second  is  the  Tscherning  theory. 

The  weak  point  in  both  theories  is  that,  for  either  increasing  or 
reducing  convexity,  the  elastic  force  of  the  lens  substance  is  a  neces- 
sary factor.  The  purpose  of  muscles  is  to  enable  one  to  overcome 
the  natural  forces,  and  in  both  directions.  Otherwise  there  is  no 
control,  or  the  control  is  one-sided.  The  antagonism  of  muscles,  so 
generally  provided  for  their  exercising  functional  power  by  alternate 
contraction  and  relaxation  in  co-operation  with  each  other,  according 
to  either  of  these  theories,  is  wanting;  and  yet  muscular  fibers  of 
the  ciliary  muscles  have  the  antagonistic  arrangement.  Is  that  ar- 
rangement without  a  purpose? 

The  Ciliary  Body. 

The  ciliary  body  is  a  triangular  ring  of  pliable  tissue,  base 
inward  and  forward  that,  at  some  distance  from  the  crystalline  lens, 


26  MUSCLES   OF  THE   EYE 

completely  encircles  it.  The  iris  is  an  appendage  from  its  anterior 
base  angle,  while  the  ciliary  processes,  which  grip  the  outer  edges 
of  the  suspensory  ligament,  are  a  feature  of  its  inner  base  angle. 
Contraction  of  the  circular  fibers  of  the  ciliary  naturally  tend  to 
sway  or  swerve  it  inward  and  backward,  or  to  cant  or  curl  it  back, 
rather  than  draw  it  toward  the  lens ;  while  contraction  of  the  radials 
or  fan-like  fibers  would  tend  to  uncurl  it  or  restore  it  to  its  former 
static  position. 

Such  a  backward  or  inward  movement  would  draw  upon  the 
anterior  capsule  of  the  lens,  and  force  it  into  the  form  Tscherning 
describes  as  its  actual  form  of  being  convexed ;  while  contrac- 
tion of  the  radials  or  fan  fibers,  would  produce  pressure  upon  the 
posterior  capsule,  and  iron  out  the  special  convexity  at  the  front 
surface,  from  which  the  tension  would  be  taken  by  relaxation  of  the 
circular  fibers.  Instead  of  being  "snubbed"  into  greater  convexity 
by  the  release  of  tension  on  it,  as  a  ship  is  snubbed  to  a  wharf  against 
or  with  the  force  of  the  current,  or  of  its  own  inertia,  it  is  muscularly 
adjusted  to  such  convexity  as  is  required  of  it,  and  by  antagonistic 
muscles,  and  the  alternating  tensions  they  apply  to  the  capsule  of  the 
lens,  and  to  the  lens  within  it. 

The  watery  character  of  the  aqueous  humor  more  readily  adapts 
itself  to  the  shape  of  the  anterior  surface  than  the  jelly-like  con- 
sistency of  the  vitreous  humor  to  any  change  of  shape  of  the  pos- 
terior surface,  which  accounts  for  the  greater  response  to  pressure 
of  the  capsule  at  the  anterior  surface.  It  is  also  more  advantage- 
ous to  have  the  change  of  curvature  at  the  anterior  surface  because 
it  is  more  effective,  on  account  of  being  at  a  greater  distance  from 
the  retina.  Since,  to  exercise  lo  D.  of  accommodation,  1 1  times  its 
static  curvature  of  lOO  meter-curves  is  necessary,  this  high  increase 
would  be  impossible  over  the  entire  anterior  surface,  but  might  be 
easily  effected  over  a  small  area  of  it. 

Motor-Nerve  Control. 

It  presents  some  difficulties  to  explain  how  the  different  muscular 
fibers  of  the  ciliary  muscles  can  be  stimulated  to  these  antagonistic 
actions,  since  there  is  no  authority  for  their  control  by  any  except 


MUSCLES   OF  THE   EYE  27 

the  3d  nerves,  and  not  by  different  systems  as  in  the  iris.  But  it  is 
possible  for  the  3d  nerves  to  be  so  distributed  to  the  fibers  as  to 
give  them  this  power.  For  example,  the  3d  nerves  supply  both  the 
superior  and  inferior  recti  muscles.  But  when  the  eye  is  turned 
upward,  the  superior  rectus  is  contracted  and  the  inferior  is  relaxed  ; 
and  the  reverse  innervation  and  action  takes  place  when  the  eye  is 
turned  downward. 

If  it  could  be  found  that  there  is  a  stimulation  of  the  fan-like 
fibers  of  the  ciliary  muscles  along  with  the  stimulation  of  the  radial 
muscles  of  the  iris,  as  there  is  such  associataion  between  the  circulars 
of  the  ciliary  and  sphincter  muscles  of  the  iris,  it  would  explain  the 
co-ordination  of  their  action.  But,  as  "there  are  things  in  heaven 
and  earth  not  explained  by  our  philosophy,"  and  optometry  is  more 
concerned  with  the  optical  effects  of  a  muscular  action  and  its  motor- 
nerve  stimulation  than  with  the  physiological  means  employed  for 
the  purpose,  we  may  pass  such  questions  up  to  those  who  specialize 
in  anatomy  and  physiology,  and  who  have  the  laboratory  facilities 
for  answering  them. 

So  far  these  higher  authorities  have  not  satisfactorily  explained 
the  control  we  have  over  the  convexing  of  the  crystalline  lens,  and 
of  reducing  its  convexity,  and,  as  this  is  but  pointing  out  the  weak- 
nesses of  existing  theories,  rather  than  offering  a  new  one,  we  can 
wait  for  it,  especially  since  the  function  of  accommodation  will  go 
on  performing  its  duties  just  as  well  as  if  we  knew  exactly  how  they 
were  performed,  and  to  the  minutest  detail. 

Sensory  Initiation. 

Any  function  is  dormant  until  some  sensory  irritation  signals  it 
to  "get  busy."  The  sensory  warning  that  there  is  need  of  greater 
or  less  refraction  in  the  dioptric  media  of  the  eye  is  the  fact  that 
vision  is  blurred,  and  how  it  is  blurred.  If  it  is  because  of  sub- 
normal acuity  of  vision,  or  because  the  media  are  not  normally  trans- 
parent, we  have  then  no  muscular  function  for  improving  it.  But, 
if  it  is  because  light  from  the  object  we  are  trying  to  visualize  is 
not  duly  focused  at  the  retina,  then  the  function  of  accommodation 
may  be  able  to  rectify  it. 


28 


MUSCLES  OF  THE  EYE 


The  non-focalization  of  light  at  the  retina  causes  diffusion  circles 
there  instead  of  point  foci,  and  each  of  these  spread  over  small 
areas  of  the  retina,  so  that  the  light  from  different  objective  points 
overlap  each  other,  causing  confusion  in  the  visual  identification  of 
details  of  the  object.  A  very  little  diffusion  obliterates  these  details 
so  that  only  the  less  detailed  general  outlines  of  objects  are  clearly 
seen,  as  all  details  must  be  of  objects  at  too  great  a  distance  from  us. 


FIGURE   7. 

Formation  and  Development  of  Diffusion  Circles  at  the  Retina:  1,  formation 
and  development  of  positive  diffusion  circles;  2,  formation  and  development  of 
negative  diffusion  circles.  In  lower  figures,  the  arrows  point  the  direction  of 
development  of  light  waves. 


Diffusion  circles  are  of  two  varieties,  those  due  to  the  intercep- 
tion of  the  light  before  it  reaches  its  focus ;  and  second  its  intercep- 
tion by  the  retina  after  it  has  reached  and  passed  its  focus.  For  the 
former,  the  rays  are  convergent,  and  the  light  v^aves  are  concave, 
and  the  waves  therefore  fall  upon  the  retina  peripherally  first  and 
develop  inwardly  to  the  axial  point;  but  for  the  latter,  the  rays  are 
divergent  and  the  waves  are  convex,  but  of  much  greater  curvature 
than  the  retina  itself,  so  that  they  reach  the  retina  at  the  center  first 
and  develop  out  to  the  periphery. 


The  visual  sense  is  able  to  differentiate  these  two  classes  of  dif- 
fusion circles.  The  sensation  of  the  first  class  indicates  that  greater 
convexity  and  refraction  is  necessary  to  bring  the  focus  to  the  retina, 


MUSCLES   OF  THE   EYE  29 

and  the  motor-nerve  stimulus  is  sent  to  the  circular  ciliary  fibers,  re- 
sulting in  increasing  the  convexity  of  the  lens  and  focalizing  light  at 
the  retina,  instead  of  back  of  it.  But  the  other  development  of  dif- 
fusion, from  a  central  point  outward,  indicates  that  less  convexity 
and  refraction  is  necessary,  and  the  stimulus  is  sent  to  the  fan-fibers 
of  the  ciliary  muscles,  to  reduce  convexity  of  the  lens,  and  so  place 
the  focus  back  upon  the  retina. 

If  it  is  doubted  that  the  visual  sense  is  able  to  recognize  these  dia- 
metrically opposite  forms  of  diffusion,  especially  in  such  intangible 
impacts  as  those  of  waves  of  light,  we  have  only  to  refer  to  what 
would  seem  equally  impossible,  its  differentiation  of  w^ave- frequency 
of  waves  that  have  a  frequency  of  many  billions  per  second,  of  which 
the  visual  sense  takes  instant  cognizance  in  seeing  colors.  The  dif- 
ference between  510,000,000,000,000  and  569,000,000,000,000  waves 
per  second  is  the  difference  between  seeing  an  object  that  is  yellow 
and  one  that  is  green  in  color.  What  is  the  purpose  of  the  fine  anat- 
omy of  the  retina  if  not  for  tasks  such  as  these? 

Motor-Nerve  Ganglia. 

The  function  of  accommodation  is  exercised  by  reflex  action,  or 
is  an  involuntary  functional  action.  There  must  be  a  motor-nerve 
center,  or  more  than  one  such  center,  for  the  purpose,  and  essen- 
tially it  may  be  said  to  preside  over  3d  nerve  activities.  But, 
whether  it  is  located  on  the  floor  of  the  fourth  ventricle  just  beneath 
the  aqueduct  of  Sylvius'  or  at  some  other  spot,  and  whether  it  alone 
exercises  this  control,  or  has  subordinates  farther  afield  to  take 
charge  of  an  individual  muscular  action  or  function,  we  can  only 
judge  by  manifestations  which  is  a  better  basis  even  than  anatomy 
and  physiology  for  forming  a  judgment. 

The  accommodation  is  called  into  action  by  the  tendency  to  diffu- 
sion circles,  but  it  only  concerns  itself  with  those  diffusion  circles 
produced  by  light  from  the  object  we  are  endeavoring  to  visualize, 
and  on  which  the  visual  sense  is  centered.  For  instance,  if  the  ob- 
ject is  a  page  of  print  at  the  usual  reading  distance,  and  the  accom- 
modation has  adapted  the  refraction  of  the  eye  to  it,  it  will  not  focus 
light  that  is  from  objects  at  a  greater  or  less  distance,  and  these  will 


30  MUSCLES   OF  THE   EYE 

form  diffusion  circles  of  both  varieties,  for  the  nearer  objects  will 
focalize  back  of  the  retina  and  the  farther  ones  will  focalize  in  front 
of  it.  We  visually  disregard  these,  and  for  them  the  reflex  is  inope- 
rative. 

But  the  instant  that  visual  attention  is  turned  from  the  object 
at  13"  to  one  that  is  farther  away,  as  at  20",  and  focused  forward  of 
the  retina,  its  diffusion  circles  become  of  visual  consequence,  and  the 
reflex  action  required  takes  place  instantly,  reducing  the  accommoda- 
tion the  I  D.  needed  to  eliminate  such  diffusion.  This  relieves  the 
muscles  that  convex  the  lens  of  their  action,  to  that  extent,  but  im- 
poses a  counteraction  upon  the  muscles  that  reduce  the  lens  convex- 
ity, so  that  diffusion  circles  of  light  from  the  objects  visualized  are 
instantly  eliminated,  but  the  object  at  13"  is  then  correspondingly 
blurred  by  diffusion  circles  of  the  opposite  kind.  This  reflex  action 
is  in  constant  use  and  activity  as  we  ply  our  daily  vocation. 

It  would  be  operated  very  inadequately  if  we  had  to  "snub"  ac- 
commodation up,  depending  upon  the  elasticity  of  the  lens  only  to 
carry  it  to  the  right  point ;  or  if  we  had  to  snub  it  back  again  for  an 
object  at  a  greater  distance.  An  automobile  that  had  to  be  slanted 
up  hill  to  provide  it  with  the  impetus  to  back  down  again;  or  that 
had  to  be  directed  downward  on  an  incline  to  get  it  to  go  ahead, 
would  not  be  considered  as  adequately  provided  with  control.  We 
know  that  the  control  of  the  accommodation  works  under  no  such 
handicap,  and  therefore  that  the  ciliary  muscles  are  provided  with 
the  means  of  convexing  or  unconvexing  the  lens,  as  required  for 
visual  purposes. 

Amplitude  of  Accommodation. 

The  utmost  power,  in  diopters,  that  the  accommodation  can  be 
made  to  exercise  is  termed  ampUtude  of  accommodation.  There  is 
a  limit  to  it  of  course,  for  the  lens,  though  of  plastic  material,  has 
its  limits  in  that  respect,  and  the  other  factors  are  not  infinite.  In 
youth  and  early  life,  the  lens  is  most  plastic  and  responds  more  fully 
to  the  tensions  that  are  applied  to  it.  But,  as  we  advance  in  years, 
this  plasticity  gradually  wanes,   and  the  amplitude   gradually   de- 


MUSCLES   OF  THE   EYE  31 

creases.  By  observation  and  comparison  a  table  of  amplitude  has 
been  prepared,  and  this  is  the  guide  to  normal  accommodation  ac- 
cording to  age. 

At  the  age  of  lo  years  the  amplitude  is  generally  observed  to 
be  about  14  D.,  but  at  the  age  of  70  years  it  is  practically  nothing. 
Between  these  points  the  reduction  takes  place  at  the  rate,  at  first, 
of  about  2  D.  in  5  years,  but  gradually  becomes  less  per  year  as  we 
grow  older.    The  accepted  table  is  about  as  follows : 

At  the  age  of 

10  14.00  D. 

15  12.00  D. 

20  10.00  D. 

25  8.00  D, 

30  6.50  D. 

35  550  D. 

40  4-50  D. 

45  350  D. 

50  2.50  D. 

55  150  D. 

60 i.oo  D. 

65 25  D. 

70  0.00  D. 

But  these  are  approximate  figures  only,  as  no  one  would  depend 
upon  them  wholly,  except  as  a  guide  in  the  determination  of  the  true 
amplitude. 

Accommodation  of  the  above  amounts  for  the  different  ages  is 
termed  normal-for-the-age.  But  there  are  other  circumstances  than 
age  to  reduce  the  amplitude.  Among  these  causes  are  the  non-use 
of  it,  for  certain  static  conditions  of  the  eyes  make  its  use  of  no 
value  for  near  vision ;  and  certain  diseases,  such  as  diphtheria,  scarlet 
fever,  grip  or  influenza  so  deplete  the  motor-nerve  power  that  the 
ciliary  muscles  are  operated  but  feebly,  and  the  amplitude  of  ac- 
commodation of  young  people  as  well  as  older  ones,  becomes  sub- 
normal for  their  ages.    But  this  loss  of  functional  power  is  usually 


32  MUSCLES   OF   THE    EYE 

restored  by  the  recovery  of  health  and  physical  vigor,  although  it  is 
sometimes  permanently  impaired. 

Another  cause  of  subnormal  amplitude  of  accommodation  for  the 
age  is  the  administration  of  certain  drugs,  either  internally  or  ex- 
ternally. If  administered  internally,  this  effect  is  not  designed,  but 
is  due  to  the  powerful  influence  of  the  drug  being  reflexed  to  the 
eyes.  But  w^hen  administered  externally,  or  directly  to  the  eyes,  its 
object  is  to  de function,  at  least  temporarily,  the  power  of  accommo- 
dation, so  as  to  enable  the  examiner  to  dispose  of  this  bothersome 
influence  while  making  an  examination  of  the  eyes.  The  drug,  in 
effect,  paralyzes  the  ciliary  sphincters,  as  well  as  those  of  the  iris, 
by  its  deadening  effect  upon  the  motor  nerves  that  supply  these  mus- 
cles ;  but  it  is,  at  the  same  time,  stimulating  to  their  antagonists. 

Optometrists  have  reasoned  it  out,  from  observation  and  expe- 
rience, that  the  use  of  these  drugs  for  such  purpose,  far  from  being 
more  than  a  temporary  advantage  to  an  unskilled  operator,  are 
really  menacing,  as  their  effects  are  apt  to  be  lasting,  and  that  they 
thus  deprive  the  eyes  of  the  function  that  is  essential  to  occupational 
vision.  They  tend  to  produce  the  same  impairment  of  the  function 
as  time  naturally  produces,  or  to  age  the  eyes,  setting  them  forward 
from  an  amplitude  pertaining  to  the  age  of  15  years  to  that  of  18  or 
20  years.  On  school  children,  whose  school  work  requires  the  ready 
adaptation  of  the  eyes  to  it,  the  eft'ect  is  really  deplorable,  although 
the  lenses  that  are  prescribed  in  co-operation  with  their  depleted  am- 
plitude of  accommodation,  restores  comfortable  near  vision  without 
impairment  of  distant  vision. 

Those  employing  the  drug  are  deceived  into  the  belief  that  it 
is  an  essential  to  fitting  the  eyes  for  distance ;  and  yet  they  will  find 
by  consulting  their  own  authorities,  that  the  method  is  not  a  depen- 
dable means  of  doing  this  work.  The  reason  for  its  non-dependabil- 
ity is  that  a  drug  of  this  kind  reverses  the  muscular  tension  of  the 
ciliary,  thus  causing  the  crystalline  lens  to  assume  an  abnormal  flat- 
ness of  form,  and  reducing  its  dioptric  value.  As  a  consequence  the 
eye  accepts  and  requires  a  positive  lens  of  greater  power  than  nor- 
mally demanded,  to  take  the  place  of  this  depletion  of  its  static  re- 


MUSCLES   OF  THE   EYE  33 

fraction.  This  is  the  real  reason  for  the  "deduction"  that  is  made  in 
all  positive  lens  findings  by  those  who  use  the  drug,  and  for  the 
natural  uncertainty  of  the  amount  of  such  deduction. 

Demand  for   Accommodation. 

The  demand  for  accommodation  with  which  to  increase  the  re- 
fraction of  the  eye,  or  to  add  to  its  positive  lens  value  by  accommoda- 
tion, rests  upon  two  points  or  conditions,  to-wit : 

1.  The  Static  or  Structural  Refraction  of  the  Eye. 

2.  The  Nearness  to  it  of  the  Object  Visualized. 

Assuming  the  eye  to  be  of  normal  refraction,  or  of  such  optical 
structure  that  the  exercise  of  accommodation  is  not  required  for  see- 
ing distant  objects  distinctly,  then  the  nearness  of  the  object  only  is 
the  sole  gauge  of  the  demand  for  accommodative  action.  When  a 
distant  object,  or  the  light  from  its  different  points,  is  focused  at  the 
retina  without  the  exercise  of  any  accommodation,  then  the  full  am- 
plitude of  that  power  is  available  for  seeing  near  objects.  This  con- 
dition is  termed  emmetropia.  In  emmetropia,  the  demand  for  ac- 
commodation is  said  to  be  normal-for-the-distance  of  the  object,  or 
merely  normal-for-the-distance.  This  is  quite  different  than  what  is 
termed  normal-for-the-age. 


13' 


FIGURE    8. 
Emmetropic    eye.    with    4D.    amplitude    of   E;Ccommodation,    fixing    object    at 
reading  distance,    13".     O,   object    at    13";    3    in   crystalline  lens,    accommodation 
in  force;  1  in  ciliary  body,  accommodation  in  reserve;  F,  focus  of  light  from  O 
at   retina. 

The  nearness  of  an  object,  or  rather  the  distance  of  a  near  ob- 
ject, is  measured  in  meters,  so  that  it  may  be  readily  reduced  to 
metric  curvature  or  diopters  by  taking  its  reciprocal.  A  meter  is 
equal  to  about  40  inches,  or  39.37  inches.  Assuming  it  to  be  40 
inches,  each  inch  is  1/40  of  39.37  =  .984  of  an  inch.     If  we  call 


34  MUSCLES   OF   THE    EYE 

this  value  i  metric-inch,  we  may  express  short  distances  in  them, 
using  the  sign  (")  to  represent  them.  Then  we  are  saved  from  the 
trouble  of  making  the  reservations  "nearly"  or  "almost"  when  giv- 
ing distances  in  them.  On  this  basis,  although  the  real  inch  has 
25.4  millimeters  in  it,  25  mm.  =  i".  It  will  be  assumed,  when  the 
above  sign  is  used,  that  metric-inches  are  referred  to,  so  that  16" 
or  20"  means  that  number  of  metric-inches. 

Accommodation  that  is  normal-for-the-distance  is  equal  to  the 
metric  curvature  of  incident  waves  of  light  from  the  object  to  the 
eye,  or  to  the  correction-plane  of  the  eye,  where  a  lens  would  be 
placed  to  neutralize  it.  Such  incident  metric  curvature  of  the  waves 
for  the  ordinary  distances  is  as  follows : 

Infinity  =  6  meters,  20  ft.  or  more,  waves     o,  ace.  o. 

I      meter  =  40"  .  waves-f-i,  ace.  i.oo  D. 

y2  meter  =  20"  waves-)-2,  ace.  2.00  D. 

y3  meter  =  practically  13"  waves-|-3,  ace.  3.00  D. 

^4  meter  =  10"  waves-f-4,  ace.  4.00  D. 

1/5  meter  =  8"  waves+5,  ace.  5.00  D. 

With  the  above  accommodation,  for  the  respective  distances  of 
the  object,  an  emmetropic  eye  will  focus  it  at  the  retina,  and  a  clear 
image  of  the  object  visualized  will  be  formed  upon  it. 

Sustained  Accommodation 

In  visually  fixing  an  object  at  any  near  distance,  the  emmetropic 
eye  must  accommodate  normal-for-the-distance.  But,  when  the  ob- 
ject is  a  page  of  reading  matter  he  is  perusing,  that  accommodation 
must  be  sustained,  for  the  moment  it  is  relaxed  or  over-exercised  the 
print,  if  not  moved  to  a  corresponding  distance,  becomes  indistrict. 
As  the  amplitude  of  accommodation,  given  on  a  preceding  page,  is 
the  utmost  power  that  it  can  exercise,  it  follows  that  this  power  can- 
not be  sustained,  but  is  only  a  momentary  exercise  of  it.  Hence,  for 
sustained  reading  at  any  fixed  near  distance,  the  whole  amplitude  can- 
not be  employed,  but  only  such  part  of  it  as  can  be  maintained  for  a 
considerable  period  of  time,  leaving  the  balance  in  reserve  for  special 
use,  if  needed. 

Some  differences  of  opinion  are  found  as  to  the  proportion  of 


MUSCLES   OF  THE   EYE  35 

the  used  to  the  reserved  accommodation  for  ordinary  near  vision  or 
reading.  It  is  the  opinion  of  the  writer  that,  as  a  standard,  but  about 
half  of  the  ampHtude  should  be  so  engaged,  although  this  may  be 
crowded  a  little  when  the  need  of  artificial  assistance  first  becomes 
manifest,  by  moving  the  printed  matter  a  little  farther  from  the  eyes. 
This  division  divides  the  amplitude  50/50  between  the  used  and  the 
reserved  portions  of  it.  Then,  by  relaxation,  more  distant  points 
may  be  fixed;  while  the  employment  of  a  little  of  the  reserve  will 
enable  one  to  see  objects  at  a  nearer  distance,  as  in  referring  to  finer 
printed  matter,  such  as  may  be  found  in  foot-notes  or  tables.  This 
we  call  the  range  of  vision  or  of  accommodation,  the  former  being 
expressed  in  distances,  the  latter  in  diopters,  but  are  reciprocally 
equivalent  to  each  other.  By  comparing  these  equivalents  in  the  above 
list  of  values,  it  will  be  seen  that  the  shorter  the  distances  the  greater 
the  dioptric  value  of  their  differences  of  distance,  or  the  less  the  dis- 
tance required  for  i  D. 

Presbyopia 

When  age  has  so  reduced  the  amplitude  of  accommodation  that, 
notwithstanding  the  fact  that  all  of  it  is  available  for  near  vision, 
none  being  required  for  distance,  it  is  insufficient  for  sustained  near 
vision  without  discomfort,  and  requires  lens  assistance,  the  condi- 
tion is  called  presbyopia.  This  is  a  deficiency  of  the  function  of  ac- 
commodation due  to  age  and  the  hardening  of  the  lens,  and  is  not  a 
muscular  deficiency.  Muscular  weakness,  due  to  illness,  is  not  pres- 
byopia, although  optically  considered,  it  may  require,  temporarily,  the 
same  means  of  relief.  Taking,  for  example,  an  emmetropic  eye  hav- 
ing the  amplitude  of  accommodation  of  4  D.  For  sustained  reading 
at  13"  it  would  require  to  accommodate  3  D.,  or  use  ^  of  its  ampli- 
tude continuously. 

That  would  soon  cause  eye-weariness,  and  to  relieve  it  the  paper 
being  read  might  be  moved  farther  away.  But  if  the  print  should  be 
rather  fine,  the  retinal  images  would  be  quite  small  and  difficult  to 
see  distinctly.  A  more  brilliant  light  might  help  matters  temporarily, 
but  that  is  not  the  real  trouble.  But  the  eyes  with  an  amplitude  of 
4  D.  can  use  2  D.  comfortably  and  continuously.  Hence,  if  both  eyes 
are  in  the  same  condition,  and  a  pair  of  -f-i  D.  lenses  are  properly 


36  MUSCLES   OF  THE   EYE 

mounted  before  them,  accommodation  for  13"  will  be  reduced  to  that 
amount,  and  a  reserve  of  2  D.  of  amplitude  will  be  provided  for  them. 
These  lenses  will  of  course  blur  distant  vision,  but  as  little  as  nec- 
essary, and  provide  comfortable  and  sustainable  vision  at  reading 
distance,  both  for  coarse  and  fine  printed  matter. 

With  the  above  lenses,  placed  at  the  correction  plane,  which  is 
at  a  distance  of  14  mm.  from  the  eye,  the  farthest  point  of  clear 
vision  is  at  the  anterior  principal  focus  of  the  lens,  40"  in  front  of  it. 
But,  as  there  is  no  accommodation  required  for  that  distance,  the  4  D. 
of  amplitude  can  be  added  to  it,  making  5  D.  in  all,  so  that  the  near- 
est point  of  distinct  vision  becomes  8"  instead  of  10".  The  lenses 
therefore  provide  the  following  range  of  vision : 

Farthest  distinct  vision 40"  ace.  0.00  D. 

Nearest  distinct  vision 8"  ace.  4.00  D. 

Comfortable  and  sustained  near  vision..  13"  ace.  2.00  D. 

Stronger  lenses  would  lessen  the  accommodation  required  for 
reading,  but  they  would  blur  distant  vision  more,  and  the  function 
of  accommodation  by  having  insufficient  exercise,  would  deteriorate 
more  rapidly. 

As  all  eyes,  whether  emmetropic  or  not,  are  assumed  to  be  first 
examined  and  corrected  for  distant  vision,  they  are  also  assumed  to 
be  made,  by  such  correction,  artificially  emmetropic,  the  above  prin- 
ciples for  correcting  presbyopia  with  lenses  apply  to  all  eyes.  But 
it  is  not  therefore  to  be  thought  that  all  eyes  follow  the  above  prin- 
ciples with  exactly  the  same  additions  or  presbyopic  lenses.  It  is 
better  to  defer  the  correction  of  this  functional  weakness  until  neces- 
sity compels  it  to  be  made,  for  the  eye  can  accommodate  no  more  and 
no  less  with  these  lenses  than  without  them,  and  the  preservation  of 
the  power  by  exercise  of  it  is  to  be  taken  into  account.  But  also, 
people  are  of  different  physical  dimensions,  some  with  long  arms  and 
some  with  shorter  ones ;  and  occupations  or  avocations  are  different, 
so  that  some  require  specially  near  vision  and  others  more  distant 
near  vision.  But  the  essential  common  principle  is  this :  Comfort- 
able and  sustained  vision  at  the  "working"  distance,  with  the  ampli- 


MUSCLES   OF  THE   EYE 


n 


tude  of  accommodation  so  divided  that,  for  greater  or  less  distances, 
it  is  equally  apportioned  in  diopters,  though  not  in  distances. 


'i* 


FIGURE  9. 
Emmetropic    eye    viewing    object    at    13"    through    a    +1    D.    spherical    lens. 
O,  object,  page  or  printed  matter;  I^,   +1  sph.  before  tlie  eye;   figure  '2'  in  lens, 
accommodation  exercised;   figure   '2'  in  ciliary  body,  accommodation   in  reserve: 
F,    focus    at    retina. 


Neurometry  of  Accommodation 

While  the  function  of  accommodation  is  exercised  directly  by 
ciliary  muscles,  the  burden  of  it  falls  finally  upon  the  motor-nerves 
that  enforce  the  action  of  the  muscles.  We  measure  the  accommoda- 
tion in  diopters,  or  physical  units,  in  which  2  D.  is  exactly  double 
I  D.,  and  all  are  in  the  ratio  of  their  numerical  expressions.  But  this 
is  quite  different  than  the  supply  of  motor-nerve  force  or  energy  with 
which  to  exercise  these  optical  effects.  Like  the  climbing  of  a  series 
of  stairways  of  exactly  equal  dimensions  throughout,  they  grow 
more  difficult  the  farther  we  ascend  them.  If  we  apply  the  common 
law  of  increase  to  this  expenditure  of  nerve  force,  as  exemplified  in 
the  law  of  friction  and  similar  things,  we  will  find  the  ratios  to  be  as 
follows : 

Assume  the  unit  to  be  that  nervous  expenditure  required  to  exert 
the  first  I  D.  of  accommodation,  or  to  flex  the  lens  the  iic  required 
amount.  This  is,  from  the  motor-nerve  standpoint,  the  easiest  di- 
opter of  all.  It  varies  with  age,  health,  etc.,  and  for  different  persons 
having  age  and  health  equality ;  but  it  is  not  intended  to  apply  it  to 
different  people.  Let  us  represent  this  expenditure  of  motor-nerve 
force  by  the  unit,  \n.  We  may  call  it  one  neuron,  or  what  we 
choose.    Then,  as  this  force  has  to  be  continued  while  we  are  nego- 


i  stance : 

Accom. 

Nerve-force 

Infinity 

0  D. 

0  n 

40" 

I  D. 

I  n 

20" 

2  D. 

4  n 

13" 

3D. 

9  n 

10" 

4D. 

16  n 

8" 

5D. 

25  n 

38  MUSCLES   OF  THE   EYE 

tiating  the  second,  increased  accommodation,  for  emmetropic    eyes, 
is  as  follows : 

Increase 

0  n 

1  n 
3  « 
5  n 
7  n 
9  n 

To  those  who  would  question  this,  or  any  similar  principle  or 
law  of  increase,  it  is  suggested  that  they  consider  the  fact  that  the 
exercise  of  increasing  power  of  accommodation  is  not  merely  the 
question  of  taking  on  more  load,  but  that,  in  negotiating  each  subse- 
quent diopter,  that  which  has  been  previously  assumed  has  to  be 
held.  One  brick  weighs  approximately  the  same  as  another  of  the 
same  kind ;  but  when  one  has  to  hold  the  two  he  has  already  picked 
up  while  picking  up  the  third,  and  the  three  while  he  is  picking  up 
the  fourth,  these  have  to  be  carried  down  and  up  in  the  excursion 
for  the  latter,  so  that,  for  the  3d,  two  go  down  and  come  back  with 
the  elevation  of  the  3d,  making  4  -J-  i  =  5  of  the  single  trips ;  and 
for  the  4th,  it  is  2  3's  =  6,  and  i  more,  making  7  in  all.  The  diop- 
ters of  accommodation  exercised  are  not  fastened  to  place  as  we 
reach  for  an  additional  one. 

But  the  strongest  indication  that  this  or  a  similar  rule  prevails 
is  the  fact  that,  when  we  supply  the  patient,  who  is  verging  on  pres- 
byopia, with  a  weak  plus  lens,  he  gets  excessive  relief  from  it  in  pro- 
portion to  its  dioptric  value.  The  reason  for  this  is  that  the  lens 
relieves  the  accommodation  of  taking  on  and  carrying  the  higher 
and  harder  diopters,  and  leaves  the  function  to  negotiate  the  easier 
ones  only.  Hence,  in  the  case  of  the  presbyope  who  is  given  a  -(-i 
lens  above,  reducing  his  accommodations  for  13"  from  3  D.  to  2  D., 
we  cut  the  motor-nerve  force  required  down  from  gn  to  4m,  or  take 
off  5n  from  the  normal  demand  for  motor-nerve  force  for  13" 
vision.  This  is  an  abatement  of  over  3^  of  that  force  he  would 
otherwise  be  compelled  to  use  for  seeing  at  the  reading  distance, 
which  is  an  abatement  indeed.  We  do  not  double  this  for  the  two 
eyes,  for  the  two  act  together  more  easily  than  one  alone. 


• 


MUSCLES   OF  THE   EYE  39 

The  Time  Element 

To  appreciate  the  motor-nerve  expense  involved  in  accommoda- 
tion in  the  higher  diopters,  for  reading,  one  must  not  neglect  to  take 
the  time  factor  into  account.  A  momentary  use  of  a  high  degree 
of  motor-nerve  energy  would  be  of  little  consequence,  but  when  this 
is  continued,  time  becomes  an  important  factor.  Reckoning  time  in 
seconds,  not  to  say  in  "split"  seconds,  as  required  in  the  exposures 
for  moving  picture  photographs,  there  are  60  of  these  in  a  minute, 
and  3600  of  them  in  an  hour.  When  we  are  reading  an  interesting 
book,  hour  after  hour  may  be  passed  away  with  scarcely  a  let  up  in 
the  accommodation. 

In  the  presbyopic  case  we  have  referred  to,  an  emmetrope  with 
an  amplitude  of  accommodation  of  4  D.,  who  needs  a  -j-i  lens  to 
relieve  it  of  excessive  accommodation,  or  the  eye- wearying  work 
involved  in  reading  at  13",  we  may  compare  its  expenditure  with  and 
without  the  assistance  of  the  lens.  The  results  of  this  comparison 
are  as  follows : 


For  13"  vision           Accommodation, 

Motor-nerve  force 

I.  Without  the  lens, 

3.00 

D. 

9 

n 

2.  With  the  lens, 

2.00 

D. 

4 
5 

n 

Excess  per  sec. 

1. 00 

D. 

n 

For  I  minute. 

I.  Without  the  lens, 

640 

n 

2.  With  lens, 

240 

n 

Excess  per  minute 

300 

n 

For  I  hour. 

I.  Without  lens, 

32,400 

n 

2.  With  lens, 

14,400 

n 

Excess  per  hour,  18,000  n 

As  a  matter  of  fact,  no  presbyopic  reader  of  this  amount  would  be 
able  to  stand  the  strain  of  such  a  wasted  expenditure  of  force.  He 
would  be  compelled,  at  short  intervals,  to  "rest  up"  the  accommoda- 


40  MUSCLES   OF   THE    EYE 

tion,  which  an  emmetrope  can  do  by  merely  visualizing  a  distant 
object,  like  an  advertisement  on  a  bill  board  across  the  street;  or  at 
night,  at  something  across  the  room ;  or  subconsciously  he  would 
hold  his  book  a  little  farther  away,  getting  relief  for  every  inch  of 
increase  in  the  distance,  but  correspondingly  reducing  his  angle  of 
vision. 


MUSCLES   OF  THE   EYE  41 

CHAPTER  IV 

Abnormal  Structures 

If  all  eyes  were  of  norinal  optical  structure,  emmetropia,  there 
would  be  little  for  the  optometrist  to  do.  His  practice  would  be  con- 
hned  to  the  fitting  of  presbyopia  and  spectacle  and  eye-glass  frames, 
which  latter  is  an  art  in  itself.  But,  as  there  are  many  eyes  whose 
optical  structure  is  abnormal  in  some  particular,  the  practice  of  op- 
tometry is  greatly  extended ;  for  distant  vision  is  impaired  or  the 
muscular  functions  are  disturbed  or  disarranged,  and  lenses  cor- 
rect these  abnormalities  or  defects. 

These  defects  of  optical  structures  are  usually  referred  to  as 
"errors  of  refraction" ;  but  why  should  an  eye  whose  lens  system 
is  of  a  perfectly  spherical  form  and  action  be  so  designated?  If  such 
action  of  the  dioptric  system  of  the  eye  is  adapted  to  its  axial  depth, 
the  eye  is  emmetropic ;  but  if  it  is  not  so  adapted  to  its  axial  depth, 
it  is  ametropia.  Hence,  neither  the  refraction,  if  spherical,  nor  the 
axial  depth  of  the  eye,  by  itself,  can  be  regarded  as  the  defective 
factor.  It  is  the  relativity  of  the  two  to  each  other  that  makes  the 
eye  either  emmetropic  or  ametropic. 

With  this  obvious  fact  in  view,  we  may  define  the  different 
optical  structures  of  the  eye  as  follows : 

1.  Emmetropia,  an  eye  whose  refraction  and  axial  depth 

are  in  normal  relationship,  or  agree  with  each  other. 

2.  Ametropia,  an  eye  whose  refraction  and  axial  depth  are    ' 

not  in  normal  relationship,  or  agreement  with  each 
other. 

These  are  structural  definitions,  as  nothing  is  said  about  the  func- 
tional efifects  of  either  the  agreement  or  disagreement  between  the 
two  factors ;  and  we  are  chiefly  concerned  with  these  functional 
effects.  To  define  them  functionally  we  must  first  differentiate  be- 
tween the  classes  of  structural  defects.  There  are  but  two  of  them : 
myopia  and  hyperopia. 

I.  In  Myopia  the  refraction  is  excessive  for  the  eye's  axial 
depth ;  or  its  axial  depth  is  excessive  for  its  refraction. 


42  MUSCLES   OF  THE    EYE 

2.  In  Hyperopia  the  refraction  is  deficient  for  the  eye's 
axial  depth ;  or  its  axial  depth  is  deficient  for  its  re- 
fraction. 

This  apparently  inverted  relationship  is  due  to  the  fact  that  the  great- 
er the  refraction  of  the  eye  the  less  must  be  its  axial  depth  to  make 
the  two  agree ;  so  that  a  strong  refraction  and  a  short  eye  are  adapted 
to  each  other,  while  a  weak  refraction  requires  a  long  eye.  An  eye 
of  any  positive  refraction  will  be  emmetropic  if  its  axial  depth  corre- 
sponds to  that  refraction. 

We  sometimes  hear  the  optical  defects  of  the  eye  explained  on 
the  basis  of  their  over  or  under  development.  This  view  of  the 
case  fails  to  consider  that,  if  an  eye  is . under-developed  and  small, 
the  metric  curvature  of  its  dioptric  surfaces  is  correspondingly  in- 
creased, which  correspondingly  increases  its  refraction,  as  the  index 
of  the  media  is  not  assumed  to  be  disturbed;  and  the  dioptric  sur- 
faces are  probably  nearer  to  each  other,  which  reduces  the  deduc- 
tion resulting  from  their  separation.  A  bird's  eye,  in  spite  of  its 
smallness,  may  be  emmetropic  or  ametropic.  It  is  not  made  hyper- 
opic  by  its  smallness ;  nor  is  an  ox's  eye,  because  of  its  largeness  or 
length,  made  myopic.  Smallness  increases  the  curvature,  largeness 
reduces  it,  so  that  refraction  is  correspondingly,  or  rather  recipro- 
cally, affected. 

Functional  Effects 

The  most  important  light  in  which  to  regard  abnormalities  of 
optical  structure  in  the  eyes  is  in  their  functional  effects  or  conse- 
quences, for  they  disturb  and  disarrange  the  muscular  function  of 
accommodation.  As  a  consequence,  some  eyes  have  near  vision  with- 
out accommodation,  or  less  than  the  normal  amount  for  the  distance 
of  the  object ;  while  others  require  to  accommodate  for  distance,  and 
more  than  normal  for  all  near  distances.  In  a  state  of  accommoda- 
tive rest,  an  ametropic  eye  will  have  either  blurred  distant  vision, 
with  good  near  vision;  or  blurred  vision  for  both  far  and  near, 
especially  for  near  vision. 

Accommodatively  considered,  the  diflferent  states  of  optical 
structure  result  as  follows : 


MUSCLES   OF  THE   EYE  43 

1.  Emmetropia  causes  accommodation  that  is  normal  for 

the  distance  of  the  object. 

2.  Myopia  lessens  the  demand  for  positive  accommodation 

at  all  distances,  but  impairs  distant  vision. 

3.  Hyperopia  increases  the  demand  for  accommodation  for 

all  distances,  and  requires  it  for  infinity. 

As  a  consequence  of  this  disarrangement  of  the  exercise  of  the  func- 
tion, the  under-use  of  it,  like  that  of  any  muscle,  leads  to  torpidity, 
or  to  dispossessing  it  of  the  power  to  act  normally,  if  called  upon  to 
do  so.  On  the  other  hand,  the  over-use  of  a  muscle,  especially  for 
continued  periods,  tends  to  develop  an  inability  of  the  muscle  to 
relax,  or  to  relieve  itself  of  its  burden;  or  to  fluctuate  in  action  so 
that  it  becomes  difficult  to  reduce  it  to  a  state  of  rest  or  quiet.  These 
eccentricities  are  sometimes  hard  to  subdue.  However,  since  the 
effects  are  as  opposite  as  the  structures,  we  must  consider  them 
separately  or  individually. 

Myopia 

Myopia  deprives  the  eye  of  clear  distant  vision,  for  the  light  it 
receives  from  distant  points  is  focused  before  it  reaches  the  retina, 
and  passing  on  from  this  point,  spreads  out  and  forms  a  diffusion 
circle  at  the  retina.  It  is  of  the  kind  that  causes  the  wave  impacts 
upon  it  to  develop  from  a  central  point  outward  to  the  periphery  of 
the  diffusion  circle,  which  warns  the  visual  sense  of  its  character. 
As  every  objective  point  in  the  distant  object  is  similarly  represented 
at  the  retina,  the  diffusion  circles  overlap  one  another,  and  the 
image  lacks  sharpness  of  definition.  This  is  a  matter  of  such  a  na- 
ture that  a  high  degree  of  visual  acuity,  instead  of  helping  it,  makes 
it  all  the  more  apparent.  We  visualize  these  images,  rather  than  the 
objects ;  the  visual  sense  has  nothing  else  to  go  by. 

It  may  be  asked  why  such  bad  focalization  is  not  neutralized  by 
that  action  of  the  ciliary  muscles  that  flattens  the  lens  and  reduces 
its  refraction  to  such  an  extent  as  place  the  foci  at  the  retina,  thereby 
clearing  distant  vision.  The  answer  to  this  question  is,  it  is  quite  a 
different  matter  to  flatten  the  lens  below  its  normal  convexity  than 
to  increase  that  convexity.     The  normal  use  of  the  radial  ciliary 


44  MUSCLES   OF   THE   EYE 

fibers  is  to  take  a  specially  acquired  convexity  out  of  the  lens,  to  re- 
duce it,  after  positive  accommodation,  back  to  its  normal  static  form, 
as  v^hen  you  straighten  a  strap  by  pulling  at  its  opposite  ends.  That 
part  of  it  is  easy.  But  when  it  is  straight,  you  may,  by  exercising 
unusual  tension,  get  it  to  yield  a  little  more  to  its  length.  This  may 
also  be  done  to  the  lens.  Statically,  or  vi^hen  at  rest,  it  is  balanced 
by  the  pressure  of  the  opposite  humors  upon  it.     It  is  easily  con- 


3" 


FIGURE  10. 
Myopia  of  2  D.  with  amplitude  of  accommodation  of  1  D.,  viewinir  a  reading 
card  at  13".     1,  rccommodation  exercised  for  object;   3,  accommodation  unused 
or  in  reserve;  F,  focus  at  the  retina,  vision  normal  for  the  distance. 


vexed,  and  restored  to  normal  form,  when  it  is  watery  and  plastic, 
as  in  youth.  But  to  reduce  its  convexity  below  that  point  can  only 
be  done  in  extreme  youth,  and  for  but  a  slight  value,  as  about  a 
single  diopter  at  I2,  and  not  more  than  1.50  D.  at  10  years.  After 
the  age  of  12  it  may  be  left  out  of  consideration,  except  as  a  neu- 
tralizer  of  accommodative  convexity. 

Inductive  Effects 

The  neuro-motor  control  of  muscles  corresponds  more  nearly 
to  electric  control  than  any  other  force,  and  corresponding  terms  are 
used  and  will  be  understood.  The  inductive  influence  of  cihary 
action  on  other  muscular  functions  is  manifest  chiefly  at  the  iris, 
and  upon  the  extrinsic  muscles.  As  myopia  gives  the  greatest  occa- 
sion for  the  functional  action  of  the  radial  fibers  of  the  ciliary  mus- 
cles, the  inductive  influence  of  this  action,  at  the  iris,  is  to  stimulate 
its  radial  muscles  by  such  influence.  As  a  consequence,  myopia  and 
a  dilated  pupil,  or  at  least  a  large  pupil,  is  a  matter  of  common  obser- 
vation. We  will  not  speak  of  the  opposite  influence  at  this  time.  It 
may  be  that  part  of  its  enlargement  is  only  apparent,  and  is  due  to 
magnification,  but  there  is  an  actual  enlargement. 


MUSCLES   OF   THE    EYE  45 

This  cannot  be  accounted  for  on  the  ground  that  the  myopic 
eye  demands  a  greater  volume  of  light,  and  therefore  its  pupil  is 
expanded.  Myopia  derives  its  name  from  the  fact  that  there  is  a 
tendency  to  close  the  lids,  and  shut  out  light,  and  for  the  purpose  no 
doubt  of  lessening  the  area  of  the  circles  of  diffusion  and  improving 
the  definition  of  images.  But  the  inductive  influence  of  the  action 
of  the  radial  fibers  of  the  ciliary  over  those  of  the  iris  is  a  stronger 
influence,  and  in  opposition  to  it.  There  is  also  an  influence  of  the 
same  general  character  on  the  external  recti  muscles,  giving  the 
myopic  eyes,  usually,  an  outward  cast,  or  exophoria.  How  the  radial 
muscles  of  the  iris,  the  fanlike  fibrils  of  the  ciliary  muscles,  and  the 
external  recti  muscles  are  nervously  connected  is  a  question  for  the 
anatomists ;  but  it  may  be  remembered  that  induction  does  not  re- 
quire contact,  or  actual  nerve  connection.  This  association  however 
tends  to  confirm  the  double  functioning  of  the  ciliary  muscles. 

Auxiliary  Effects 

The  special  activity  of  the  fan-fibers  of  the  ciliary  muscles  in 
myopia  is  confirmed  by  a  number  of  further  facts  of  a  corroborative 
character.  One  of  these  is  the  noticeable  development  of  these 
fibers,  in  comparison  with  the  circulars,  in  the  post  mortem  examina- 
tion of  the  eyes  of  a  myope.  In  emmetropia,  and  especially  in 
hyperopia,  these  muscles  are  thin  and  tendonous,  while  the  circulars 
have  the  opposite  character.  But  this  evidence  of  little  use  in  emme- 
tropes  and  hyeropes  does  not  materialize  in  myopes,  although  the 
circulars  are  always  considerably  more  developed,  as  they  are  the 
more  essential  or  more  direct  agents  of  accommodative  activity,  and 
the  fan-fibers  function  principally  as  checks  to  them. 

As  these  ciliary  fibers  extend  toward  or  to  the  apex  of  the  tri- 
angular shaped  ciliary  body,  if  not  into  the  choroidal  tissue  back  of 
it,  their  contraction  has  a  stretching  effect  upon  the  choroid,  and 
tends  to  draw  it  away  from  its  normal  attachment  at  the  optic  nerve- 
head,  or  disc,  which  accounts  for  the  white  ring  or  crescent  that 
surrounds,  partly  or  wholly,  the  optic  disc,  which  is  a  characteristic 
of  myopia.  It  may  thin  out  or  cause  slight  posterior  staphyloma,  ac- 
counting also  for  increasing  or  "progressive"  myopia,  if  the  defect  is 


46  MUSCLES   OF  THE    EYE 

not  corrected  early  and  in  full,  or  as  nearly  full  as  possible  for  com- 
fortable wearing  at  the  time  the  correction  is  made. 

Myopic  asthenopia  is  another  indication.  While  for  myopia  of 
a  degree  that  cannot  be  so  neutralized  for  distance,  even  by  a  young 
person,  it  will  only  be  employed  to  read  at  a  distance  slightly  beyond 
the  static  far  point,  for  within  the  far  point  distance  the  positive  ac- 
commodation is  employed,  and  asthenopia  is  unlikely,  except  in  case 
of  astigmatism.  But  a  low  degree  of  myopia  in  quite  young  people, 
which  offers  results  from  this  ciliary  action,  the  strain  of  it  may  be 
such  as  to  make  distant,  or  even  near  vision,  painful  and  uncomfort- 
able, indicating  that,  although  the  functional  action  takes  place,  it  is 
not  one  to  be  enjoyed.  A  weak  plus  lens,  in  such  cases,  relieves  the 
asthenopia,  but  impairs  distant  vision  much  in  excess  of  its  dioptric 
value.  It  is  explained  by  the  fact  that  it  so  increases  the  myopia  as 
to  take  it  beyond  the  range  of  this  functional  action. 

The  other  important  corroborative  fact  we  have  already  re- 
ferred to,  to-wit :  the  myotic  effect  of  a  drug  used  to  pacify  or 
paralyze  ciliary  action.  The  drug  undoubtedly  has  that  effect  upon 
the  circular  fibers  of  the  ciliary,  but  it  is  not  needed  in  myopia.  Its 
effects  upon  other  ciliary  fibers,  corresponding  to  its  effects  upon  the 
radial  muscles  of  the  iris,  is  to  stimulate  them  to  activity,  or  to  force 
them  to  so  act  as  to  flatten  the  lens,  even  below  normal  static  form. 
This  action  is  effective  in  reducing  the  apparent  myopia,  but  only  for 
a  small  part  of  its  static  refraction — not  enough  to  warrant  the  use 
of  such  heroic  measures.  If  the  myopia  is  slight,  the  eye  might  be 
made  to  appear  emmetropic,  or  actually  turned  into  hyperopia  by  it. 
However,  these  strenuous  effects  are  no  longer  sought  by  the  drug 
users,  and  a  milder  distillation  of  a  less  powerful  cycloplegic  is  con- 
sidered to  answer  the  purpose,  which  is  mainly  psychological. 

Far  and  Near  Points 

The  limit  of  a  myope's  distant  vision  is  the  reciprocal,  in 
meters  of  the  myopia.  This  is  obtained  by  dividing  loo  by  the 
myopia  for  the  distance  in  centimeters,  or  40  by  the  same  for  the 
distance  in  metric-inches.    As  it  is  forward  of  the  eye,  it  is  termed  a 


MUSCLES   OF   THE   EYE  47 

plus  distance.  For  4  D.  of  myopia  this  far  point  is  at  a  distance  of 
100/4  =  25  cm. ;  or  40/4  ■=  10".  No  accommodation  is  required, 
and  none  may  be  exercised,  for  the  focalization  of  light  from  that 
distance.  Therefore,  the  full  amplitude  of  the  accommodation  is 
available  for  the  focusing  of  light  from  a  nearer  point.  If  the 
amplitude  of  accommodation  for  the  4  D.  myope  is  6  D.,  the  near 
point  is  found  by  taking  the  reciprocal  of  the  sum  of  the  two, 
myopia  and  amplitude,  or  4  -[-  6  =  10,  the  reciprocal  of  which  is 
10  cm.  or  4".  The  range  of  vision  is  then  from  4"  to  10",  or  from 
ID  cm.  to  25  cm.,  or  vice  versa. 

The  range  of  vision  has  to  be  stated  by  giving  the  two  extremes 
of  distance,  near  and  far,  of  distinct  vision.  It  cannot  be  stated  by 
giving  the  distance  between  the  two.  In  the  above  myopic  case  of 
4  D.,  with  an  amplitude  of  6  D.,  the  space  from  the  near  point  to  the 
far  point  is  but  10  —  4  =  6".  But  we  would  be  very  indefinite  if 
we  merely  stated  it  to  be  six  inches.  If  the  near  point  of  an  eye  is 
10''  and  the  far  point  is  16",  this  also  is  a  difference  of  6'',  but  how 
different  the  two  eyes  are  in  range  of  vision  and  in  amplitude  and 
optical  condition.  The  latter  eye  is  2^/2  D.  myopic,  and  has  an  ampli- 
tude of  accommodation  of  1J/2  D.  That  is,  the  far  point  at  16'' 
shows  2}^  D.  of  myopia,  and  since  the  near  point  is  at  10",  represent- 
ing 4  D.  of  normal  accommodation,  the  real  amplitude  is 
4  D.  —  23/^  D.  =  i^  D.  We  must  know  where  the  six  inches  is 
located  to  determine  what  the  amplitude  and  refraction  are,  the 
abstract  distance  between  the  far  and  near  points  tells  us  nothing. 

The  far  point  is  termed  the  punctum  remotum,  or  p.  r.,  and  the 
near  point  is  termed  the  punctum  proximum,  or  p.  p.  The  punctum 
comfortable,  or  p.  c.  is  usually  placed  in  a  particular  position  by  the 
lenses  that  we  prescribe,  for  without  the  lenses  we  are  not  apt  to 
have  any.  For  example,  in  the  case  of  the  4  D.  myope,  with  6  D. 
accommodation,  he  may  be  able  to  read  comfortably  at  his  far  point, 
but  can  see  distinctly  there  only  with  a  fully  relaxed  accommodation. 
For  nearer  distances  only,  he  draws  upon  it.  This  is  too  near,  for 
although  the  accommodation  is  relaxed,  he  must  converge  the  two 
eyes  to  that  distance  to  have  binocular  single  vision  of  it,  and  that 
may  be  an  effort,  besides  introducing  other  complications  that  cannot 
be  discussed  here. 


48  MUSCLES    OF  THE   EYE 

But,  as  we  are  not  considering  the  procedure  in  fitting  the  eye 
or  eyes  with  lenses,  but  merely  their  optical  conditions  and  functions, 
it  would  be  outside  our  purpose  to  extend  the  discussion  any  far- 
ther than  necessary  for  the  purpose. 

Presbyopia 

The  myope  becomes  a  presbyope  in  due  course  of  time,  probably 
at  an  earlier  age  than  an  emmetrope  or  hyperope,  on  account  of  the 
function  of  accommodation  being  undeveloped  by  non-use,  unless  he 
is  corrected.  His  myopia,  as  if  it  should  be  2  D.,  would  postpone  the 
time  when  reading  lenses  would  be  necessary  to  the  neighborhood 
of  55  years,  or  to  the  age  when  an  emmetrope  requires  -\-2  lenses 
for  his  presbyopia.  When  the  emmetrope  begins  to  wear  -\-2  lenses 
for  reading,  he  converts  himself  into  a  2  D.  myope  with  the  lenses, 
or  makes  the  eyes  artificially  of  that  optical  condition.  He  is  blurred 
2  D.  for  distant  vision  the  same  as  a  2  D.  myope,  and  he  can  read 
at  13"  with  I  D.  of  accommodation  the  same  as  the  myope. 

But,  should  the  2  D.  myope  be  corrected  for  distance  in  early 
life,  so  that  the  accommodative  function  would  be  developed  nor- 
mally, his  near  vision  would  fail  at  about  the  same  time  as  that  of  a 
natural  emmetrope.  Presbyopia  is  corrected  by  myopia,  not  elim- 
inated by  it.  A  myope  of  an  amount  exceeding  3  D.  must  wear 
minus  lenses  for  comfortable  and  normal  vision  at  13",  for  his  far 
point  is  nearer  to  the  eye  than  this.  A  6  D.  myope,  in  order  to  see 
at  13"  must  wear  at  least  — 3  lenses.  If  his  lenses  are  — 6,  to  correct 
his  myopia,  he  must  accommodate  normally  for  the  distance,  as  these 
lenses  make  him  artificially  emmetropic.  If  — 5  lenses  are  the  appro- 
priate ones  for  such  reading  distance,  these  may  be  considered  to 
represent  the  — 6  lenses,  with  -\- 1  added  for  presbyopia.  Nobody  can 
escape  presbyopia  with  his  life. 

In  myopia  as  in  emmetropia,  the  due  correction  of  presbyopia 
must  require  a  plus  lens,  in  as  much  as  it  replaces  statically  a  plus 
functional  power.  But  what  the  amount  of  the  presbyopia  is,  is  rela- 
tive to  the  static  or  structural  refraction  of  the  eye.  Two  men  may 
wear,  each  — 2  lenses  for  reading  at  13".  That  does  not  indicate  that 
their  eyes  are  alike.  One  may  be  a  myope  of  3  D.  but  with  I  D.  of 
presbyopia;  the  other  a  4  D.  myope,  but  with  2  D.  of  presbyopia. 


MUSCLES   OF  THE   EYE  49 

If  their  mountings  fitted  each,  either  might  wear  the  other's  glasses 
for  reading;  but  if  they  were  converted  into  bifocals,  neither's  would 
do  for  the  other.  And  a  lost  or  detached  scale  from  a  lens  of  one 
pair,  would  not  answer  for  either  lens  of  the  other  pair.  Besides,  if 
the  distance  corrections  and  the  additions  were  the  same,  the  scales 
of  one  might  not  fit  the  others  on  account  of  being  ground  on  differ- 
ent curves. 


4 


1? 


FIGURE  11. 

Hyperope  of  2  D.  with  amplitude  of  accommodation  of  5  D.,  endeavoring  to 
read  at  13".  O,  object;  4  accommodation  exercised  (not  sufficient);  1.  accom- 
modation in  reserve   (exhausted);   P,   focus   1  D.  back  of  retina,   blurred  vision. 


Hyperopia 

Hyperopia  of  a  limited  amount  does  not  impair  distant  vision, 
when  the  accommodation  is  of  sufficient  amplitude  to  neutralize  it 
for  that  purpose.  The  eye  focuses  the  light  from  distant  points  by 
exercising  accommodation  equal  to  its  hyperopia.  It  must  maintain 
this  amount  continuously  to  see  distant  objects  clearly,  however. 
Hence,  with  age,  and  the  consequent  loss  of  accommodative  power, 
the  time  comes  when  even  vision  of  distant  objects  is  impaired  by  it. 
It  is  this  possession  of  distant  vision  by  the  hyperopic  eye  that 
led  to  its  being  named  "far  sighted"  from  which  the  term  "hyper- 
opia" is  also  derived.  The  age  at  which  distant  vision  is  impaired 
depends  upon  the  amount  of  hyperopia. 

With  a  rather  low  degree  of  hyperopia,  near  vision  may  be 
maintained  for  a  number  of  years,  as  well  as  distant  vision.  But, 
since  near  vision  requires  more  accommodation  than  distant  vision, 
near  vision  is  impaired  long  before  there  is  any  apparent  impair- 
ment of  distant  vision;  and  it  is  this  maintenance  of  distant  vision, 
after  near  vision  has  long  been  impaired  that  causes  it  to  be  referred 
to  as  above  or  by  the  corresponding  term,  "long  sighted".  When 
the  near  vision  of  a  person  begins  to  be  impaired  at  a  rather  early 


so  MUSCLES    OF   THE   EYE 

age  for  presbyopia,  but  distant  vision  remains  normal,  it  is  pretty 
surely  because  the  person  has  hyperopia  of  a  greater  or  less  amount. 
The  age  at  which  presbyopia  really  begins  in  normal  eyes  is  at  about 
the  age  of  38  years,  although  glasses  are  not  usually  prescribed  for 
it  until  the  age  of  43  or  45. 

Let  us  consider  the  case  of  a  person  of  35  years  of  age,  with 
but  I  D.  of  hyperopia,  the  eyes  being  otherwise  normal.  At  that  age 
there  is  an  amplitude  of  about  5.50  to  6  D,  of  accommodation.  This 
hyperope  must  accommodate  i  D.  for  distant  vision,  using  this 
easiest  of  all  diopters  of  his  accommodation  for  it.  For  reading 
at  13"  he  must  employ  3  D.  more,  or  4  D.  in  all,  to  see  the  type 
clearly.  This  is  considerably  over  half  of  his  amplitude,  and  the 
eyes  will  weary  under  its  continuous  use.  But,  as  is  universal  with 
hyperopes,  he  will  stoutly  maintain  that  his  eyes  are  "just  as  good 
as  ever".  Why  shouldn't  he?  He  can  see  perfectly,  both  near  and 
far.  Glasses,  why  does  he  need  glasses?  That  is  the  layman's  point 
of  view,  and  is  held  universally,  even  by  some  college  professors. 
Glasses  are  for  those  who  need  them,  not  for  such  as  he. 

Further  Examination 

Perhaps  the  above  patient  suffers  some  lapse  of  health,  per- 
haps he  has  an  unusual  amount  of  near  work  to  do,  or  perhaps  some 
dimness  that  he  observes  in  his  vision,  after  a  long  period  of  night 
reading  with  an  unsatisfactory  light  (lights  become  unsatisfactory 
to  him  at  about  this  time).  An  examination  shows  the  i  D.  of 
hyperopia  manifest.  He  just  desires  the  glasses  to  help  him  along 
with  some  special  work,  and  gets  the  prescription  filled.  They  fill 
his  requirements  exactly  and  he  goes  at  his  job  with  them  on,  taking 
them  off  as  soon  as  he  is  through,  as  there  is  "no  further  need"  for 
them.  There  are  probably  many  thousands  of  people,  counting  in 
the  university  professors,  the  doctors  and  the  lawyers,  the  school 
teachers,  the  nurses  and  the  thousands  of  others  at  this  age  that  this 
description  fits. 

But  by  mere  accident  or  experimentally,  the  wearer  neglects  to 
take  them  off  when  he  or  she  stops  near  work.  They  may  slightly 
blur  distant  vision  at  first,  but  they  feel  comfortable,  and  soon  they 
become  a  most  desirable  "easement"  to  their  eyes.    Both  distant  and 


MUSCLES   OF  THE   EYE  51 

near  vision  is  now  perfect  and  comfortable.  In  fact  it  is  rather  dis- 
agreeable to  be  without  them.  Our  Mr.  Brown  or  Miss  Jones  has 
become  a  perennial  spectacle  wearer,  and  why?  Oh,  the  "habit" 
has  been  formed ;  the  glasses  have  made  themselves  essential,  and  the 
"doctor"  who  fitted  them  is  responsible  for  the  consequences,  for 
before  they  were  worn  there  was  no  need  of  them ;  and  that  is  the 
layman's  view  again. 

But,  there  is  another  view  of  the  situation,  a  "prejudiced"  one 
of  course,  that  the  i  D.  hyperope  cannot  get  a  pair  of  +i  D.  lenses, 
in  any  form  of  mounting,  before  his  eyes  too  soon.  That  i  D.  of 
hyperopia,  uncorrected  by  lenses,  makes  i  D.  of  accommodation 
necessary  all  of  the  time,  and  adds  i  D.  to  the  accommodation  re- 
quired for  near  vision  at  any  distance.  To  the  3  D.  normally  de- 
manded for  ordinary  reading,  this  added  diopter  is  the  4th.  For  any 
distance  of  the  object,  there  is  never  a  complete  relaxation.  It  is 
quite  easy  for  one  who  is  young  and  healthy  to  maintain  i  D.  of 
accommodation,  especially  the  easiest  one  in  whatever  series  of  diop- 
ters of  power  the  eyes  have.  But,  spending  that  power  on  distant 
vision,  wasting  it,  leaves  the  harder  ones  for  necessary  use  in  near 
vision;  and  in  motor-nerve  force,  the  price  gets  higher  all  the  way 
up,  and  is  greatest  at  the  4th  if  one  goes  no  higher. 

A  glance  at  the  "neurometry"  of  it  will  be  helpful,  that  is,  will 
help  to  impart  an  understanding  of  it.  The  neurometry  involved  in 
I  D.  of  hyperopia  may  be  displayed  as  follows : 

Motor-nerve  Force 
I  n. 
4  n. 
9  n. 

16  n. 

25  n. 

The  reason  for  including  distances  nearer  than  the  normal  13" 
reading  distance  is  that  anyone,  reading  at  that  distance,  will  occa- 
sionally be  required  to  scrutinize  a  poorly  printed  word  or  the  finer 
type  of  any  foot  note  at  a  nearer  distance,  so  that  a  distance  of  10'' 
is  frequently  necessary  to  make  the  print  clear.    But,  for  vision  at 


Object  distance 

Accommodation 

Infinity 

I  D. 

40- 

2  D. 

20'' 

3D. 

ir 

4D. 

10'' 

5D- 

52  MUSCLES   OF   THE    EYE 

13''  the  motor- nerve  force  required  is  i6n,  and  this  is  16  —  g  =  yn 
more  than  normal,  or  the  amount  required  of  emmetropia  for  the 
same  distance. 

Whatever  may  be  said  of  it,  or  the  necessity  of  wearing  lenses 
to  correct  i  D.  of  hyperopia,  it  is  obvious  that  the  want  of  such  cor- 
rection causes  the  expenditure  (waste)  of  a  rather  large  amount  of 
nervous  energy,  as  it  is  this  motor-nerve  energy  that  operates  the 
ciliary  muscles  by  which  the  required  accommodation  is  effected.  And 
the  store  of  motor-nerve  energy  in  the  body,  or  its  generation  for 
motor  purposes,  is  not  confined  to  this  purpose.  It  serves  all  de- 
mands of  the  organs  of  the  body,  heart,  lungs,  Hver,  kidneys,  stom- 
ach, intestines,  and  even  the  exercise  of  the  mental  faculties.  A  pair 
of  eyes  of  this  kind,  therefore,  represent  a  serious  leakage  of  nervous 
energy,  motor-nerve  force.  One  cannot  say  what  consequences  are 
most  likely  to  follow.  Perhaps  it  will  be  nothing  perceptible,  but 
that  is  not  likely.  At  least,  sooner  or  later,  it  will  manifest  itself 
at  the  eyes  in  impaired  vision. 

Finding  Hyperopia 

With  perfect  distant  and  near  vision,  hyperopia  is  concealed 
from  direct  observation.  Myopia  is  open ;  no  one  can  hide  it ;  it  im- 
pairs distant  vision.  But  hyperopia,  which  is  covered  by  ciliary  ac- 
tion or  accommodation,  must  be  searched  out.  We  cannot  look  into 
the  eye  and  see  it,  although  by  impairing  vision  of  the  fundus  in 
ophthalmoscopy,  it  may  indicate  its  presence  to  an  expert  observer. 
A  distant  vision  test  will  not  disclose  it ;  nor  will  a  near  vision  test, 
unless  the  person's  age  is  taken  into  consideration,  or  accounts  for 
the  failure  of  near  vision.  Even  then  we  only  guess  what  part  of  the 
impairment  is  due  to  hyperopia,  and  assign  the  rest  to  presbyopia ; 
or  we  had  better  guess,  if  we  guess  at  all,  what  part  of  it  is  pres- 
byopia, and  assign  the  rest  to  hyperopia,  as  the  age  alone  will  afford 
us  a  key  to  guessing  the  presbyopia  with  a  greater  degree  of  assur- 
ance than  we  can  guess  the  hyperopia. 

But  these  would  not  be  findings,  as  there  is  nothing  certain  with- 
in our  grasp.    We  must  test  the  eyes,  either  subjectively  or  objec- 


MUSCLES   OF  THE   EYE  53 

tively,  to  ascertain  the  facts.  Subjectively  this  is  done  by  ascertain- 
ing what  weakest  plus  lens  blurs  distant  vision,  or  what  strongest  plus 
lens  has  no  such  effect.  The  objective  method  is  to  shadow  test  the 
eye,  which  is  the  most  complete  objective  method  of  measuring  its 
optical  condition,  measuring  its  refraction,  (relative  to  its  axial 
depth)  as  required.  Without  going  into  the  details  of  either  of  these 
methods,  we  consider  the  case  from  the  visual  standpoint,  as  that  is 
the  final  arbiter  of  the  facts  we  can  depend  upon,  although  it  may 
take  us  some  time  to  get  at  the  bottom  facts  even  by  this  method, 
but  the  superficial  ones  are  easy. 

Subjective  Findings 

If  an  eye  is  hyperopic,  for  a  person  not  so  far  along  in  life  that 
the  accommodation  has  become  inoperative,  and  also  if  the  hyper- 
opia is  not  too  great  to  be  neutralized  for  distant  vision,  the  eye  will 
exercise  the  necessary  accommodation  to  see  distant  objects  dis- 
tinctly. As  the  action  is  involuntary,  the  accommodative  instrumen- 
talities, sensory  and  motor,  act  for  themselves  without  consulting 
us  about  it.  When  the  sensory  requirement  exists  the  motor  re- 
sponse takes  place.  Therefore,  to  put  a  stop  to  it  at  the  "fountain 
head"  the  sensory  initiative  must  be  eliminated,  and  a  plus  spher- 
ical lens  before  the  eye  is  the  agent  we  employ.  If  the  eye  is  i  D. 
hyperopic,  or  if  it  accommodates  i  D.  while  viewing  distance,  a  -j-  i 
sph.  before  the  eye  makes  that  action  no  longer  necessary,  and  the 
accommodation  is  relaxed. 

The  accommodation  is  thus  forced  to  relax  by  the  same  method 
that,  without  the  lens,  it  is  forced  to  act.  If  it  continued  to  act  after 
the  lens  was  placed  before  it,  it  would  focus  light  from  distance  for- 
ward of  the  retina,  and  the  sensory  warning  of  this  causes  the  nerve 
center  to  withdraw  stimulation  from  the  contracting  muscle  and  send 
it  to  the  opposing  muscles.  Both  are  therefore  reflex  actions  and 
involuntary.  Unless  distant  vision  is  impaired  by  the  lens,  we  know- 
that  accommodation  has  been  relaxed.  To  be  sure  as  to  the  quan- 
tity, we  place  a  plus  lens  of  sufficient  dioptric  value  before  the  eye 
to  impair  distant  vision,  or  fog  distant  vision  with  a  plus  lens.  Then, 
by  reductions  we  obtain  the  strongest  plus  lens  that  will  not  impair 


54  MUSCLES   OF   THE   EYE 

distant  vision,  and  this  lens  measures  the  amount  of  the  accommoda- 
tion that  is  relaxed  or  abated,  and  therefore  the  manifest  hyperopia. 

The  above  method  of  procuring  accommodative  relaxation,  while 
the  vision  is  fixed  upon  distance,  and  thus  determining  the  amount 
of  hyperopia,  is  the  initial  work  of  what  is  known  as  the  fogging 
system  of  suspending  the  accommodation.  Unless  the  eye  is  accom- 
modating for  distance  it  cannot  be  relaxed  with  a  plus  lens.  If  it  is 
hyperopic  and  does  not  accommodate  for  distance,  distant  vision  will 
be  blurred  without  the  lens  but  the  lens  will  improve  it.  Therefore 
this  way  of  uncovering  accommodative  action,  and  of  neutralizing 
it  or  replacing  it  by  a  lens,  for  the  purpose  of  making  the  hyeropia 
apparent,  and  measuring  the  amount  of  it,  is  the  principal  subjective 
staff  upon  which  optometry  depends  to  determine  optical  defects  of 
the  eyes  that  are  hyperopic.  Myopic  eyes  are  fogged  by  their  myopia. 


FIGURE  12. 

Ciliary  eccentricities.     S,  static  form  of  lens;  L,  latent  or  unrelaxable  con- 
vexity of  lens;  M,  manifest  or  relaxable  convexity  of  lens. 


Ciliary  Eccentricities 

It  would  seem,  without  astigmatism,  that  the  correction  of  eyes 
with  lenses  were  a  very  simple  matter.  Myopia  is  corrected  by  the 
weakest  minus  sphere  that  gives  the  best  vision;  hyperopia  by  the 
strongest  plus  sphere  that  does  not  impair  distant  vision;  presby- 
opia by  the  weakest  plus  addition  to  the  distance  correction  that  pro- 


MUSCLES   OF  THE   EYE  55 

vides  a  suitable  range  of  vision  in  opposite  directions  from  the  p.  c, 
2/3  toward  the  p.  r.  and  1/3  toward  the  p.  p.,  and  half  of  the  ampli- 
tude of  accommodation  be  engaged  when  using  the  eyes  for  reading 
or  working,  or  for  vocational  near  vision. 

In  myopia  there  are  no  ciliary  complications,  except  the  possible 
one  of  weak  myopia  in  young  children,  below  the  age  of  12,  in  which 
it  may  be  neutralized  by  ciliary  tension  that  slightly  flattens  the 
lens.  In  that  case  it  may  be  better  to  prescribe  a  weak  plus  lens, 
thus  eliminating  myopic  asthenopia,  and  impairing  distant  vision 
but  slightly,  than  to  take  the  chance  of  prescribing  a  minus  lens  that 
does  not  improve  distant  vision.  But,  with  the  relatively  strong 
and  much  used  circular  ciliary  fibers,  the  case  is  different,  quite. 
These  muscles,  although  comparatively  small,  are  subject  to  the  same 
influences  and  show  the  same  effects  as  other  muscles  that  are  sub- 
ject to  continuous  tension  or  strain.  The  heart,  by  its  beatings,  rests 
between  beats,  and  at  least  1/3  of  the  time.  Other  organs  have  their 
periods  of  rest  distributed  to  them  with  as  great  regularity  as  their 
working  periods.  But  the  hyperopic  eye  can  have  ciliary  rest  only 
when  we  sleep. 

Continuous  contraction  of  a  muscle,  with  occasional  extra  ten- 
sions may  cause  it,  in  weariness,  to  fluctuate.  If  you  are  already 
bearing  a  load,  you  are  more  uncertain  or  unsteady  in  applying  an 
added  tension  to  the  muscles  than  you  would  be  in  starting  from  a 
point  of  rest;  and  if  you  hold  the  weight  for  some  time,  you  find  it 
difficult  to  fully  relax  the  muscles  that  held  it.  The  ciliary  muscles 
may,  by  strain,  suffer  a  sort  of  palsy,  causing  them  to  fluctuate.  They 
are  also  subject  to  spasm  or  cramp,  or  inability  to  relax  after  being 
under  continuous  tension,  as  in  hyperopia.  The  house-painter's  fin- 
gers, that  have  held  the  handle  of  a  brush  all  day  and  plied  it  back 
and  forth,  cannot  be  straightened  again  at  night.  That  is  spasm  or 
cramp,  and  he  would  not  be  steady-handed  enough  to  write  a  letter 
or  thread  a  fine  needle.  The  ciliary  is  not  different  than  other  mus- 
cles in  this  respect. 

When  the  ciliary  muscles,  the  muscles  that  flex  the  lens  for  the 
exercise  of  positive  accommodation,  get  into  a  state  of  cramp  from 
continuous  contraction,  as  exacted  of  them  in  hyperopia,  they  are  not 


56 


MUSCLES  OF   THE   EYE 


going  to  be  persuaded  to  relax  by  a  plus  lens  that  makes  such  con- 
traction no  longer  necessary.  Therefore  the  real  hyperopia  of  the 
eye  may  be  covered  up  or  concealed,  and  is  then  termed  latent;  while 
the  part  that  is  relaxed  by  a  lens  is  termed  manifest.  There  is  nat- 
urally an  element  between  the  two,  a  sublatent  or  slow-manifest 
amount  that  has  to  be  coaxed  to  come  forth  and  show  itself,  or  to 
surrender  finally  to  the  action  of  a  stronger  lens.  These  elements 
are  independent  of  what  is  termed  the  normal  tone  or  tonicity  of  a 
muscle,  indicating  a  readiness  on  its  part  to  respond  to  stimulation. 
Hence,  the  partial  spasm  that  may  be  drawn  out  is  termed  a  clonic 
spasm,  and  that  that  will  not  yield  is  termed  a  tonic  spasm.  We  thus 
have  these  hidden  influences  to  deal  with  in  correcting  hyperopia  in 
full.  This  influence  also  extends  to  the  extrinsic  muscles,  which 
cannot  be  considered  at  this  time. 

The  medical  practitioner  has  been  prone  to  magnify  these  con- 
cealed factors ;  but  the  optometrist  is  apt  to  go  as  far  in  the  oppo- 
site direction,  and  to  ignore  them  completely.  There  is  a  middle 
ground. 


FIGURE  13. 
Eye  with  2  D.  compound  hyperopic  astigmatism  "with  the  rule"  (.50  D.  in 
vertical,  2.50  D.  in  horizontal);  amplitude  of  accommodation,  6  D.;  .50  D.  ac- 
commodation in  use;  5.50  D.  accommodation  in  reserve;  V,  focus  upon  retina; 
H,  focus  2  D.  back  of  retina.  Horizontal  lines  of  astigmatic  chart  distinct. 
Vertical  lines  blurred. 


Repression 

Any  means  that  may  be  employed  to  relax  the  ciliary  muscles, 
subject  to  the  influences  just  described,  is  called  suspension,  repres- 


MUSCLES   OF  THE   EYE  57 

sion  or  suppression  of  the  accommodation.  Its  purpose  is  to  put  the 
ciliary  muscles  in  a  complete  state  of  relaxation,  so  that  the  crystal- 
line lens  will  assume  its  static  form,  or  be  at  an  equilibrium  between 
the  opposite  pressures  of  the  humors  that  surround  it,  and  have 
no  special  convexity,  but  be  of  its  normally  convex  form.  As  a  semi- 
liquid  or  humor,  the  lens  has  elasticity,  for  that  is  a  general  prop- 
erty of  liquids  and  of  matter.  But  it  is  nonsense  to  suppose  that,  in 
a  state  of  rest,  there  is  any  special  tension  upon  it,  due  to  "draw" 
of  the  suspensory  ligament  when  the  ciliary  muscles  are  relaxed. 

The  medical  practitioner  calls  his  method,  the  use  of  a  drug, 
"suspension''  of  the  accommodation.  It  would  be  more  proper  to 
term  such  a  method  "repression"  or  "suppression",  as  it  is  by  the 
means  of  influences  not  of  an  optical,  but  chemical  character,  a 
method  of  chemical  violence  that  is  allowable  for  surgical  or  thera- 
peutic purposes,  but  hardly  so  for  so  simple  a  matter  as  fitting  the 
eyes  with  lenses.  The  optometrist  is  limited,  both  professionally 
and  ethically,  to  the  employment  of  optical  means  only,  the  use  of 
lenses  for  the  purpose.  But  these  are  the  natural  and  the  best  means 
of  restoring  the  accommodation  to  normal  action  and  relaxation. 
They  never  do  more  than  merely  relax  accommodation.  The  word 
"suspension"  is  the  legitimate  word  for  optometry.  It  releases  cili- 
ary tension  by  taking  the  ground  out  from  under  it. 

When  a  ciliary  spasm  is  indicated,  both  by  a  less  amplitude  of 
accommodation  than  normal-for-the-age,  by  fluctuations  of  ciliary 
action,  causing  alternate  clear  and  dim  vision,  or  inability  to  main- 
tain steady  near  vision  at  any  accommodative  distance,  the  optome- 
trist must  employ  methods  that  are  best  calculated  to  release  the  cili- 
ary from  spasm.  He  should  at  least  correct  all  of  the  manifest 
hyperopia,  and  go  into  the  fog  as  far  as  he  deems  it  advisable,  with 
the  given  patient,  and  feel  assured  that  the  lenses  will  be  worn,  as 
directed,  constantly.  Corroborative  signs  of  latent  hyperopia  are 
various,  but  a  tendency  to  esophoria  (apparently  real  esophoria)  is 
one  of  the  strongest  indications.  The  ciliary  cramp  may  yield  slowly, 
or  it  may  yield  quickly,  to  a  plus  fogging  method ;  but  it  will  never 
yield  more  than  the  full  value  of  the  cramp.  There  are  no  deductions 
to  be  made  from  the  latest  finding  of  a  greater  for  a  less  plus  value. 


58  MUSCLES   OF  THE    EYE 

Astigmatism 

Astigmatism  is  but  a  combination  of  different  refractive  con- 
ditions in  tbe  same  eye,  or  in  different  meridians  of  it,  regularly  and 
relatively  at  right  angles  to  each  other.  Both  may  be  hyperopic, 
with  one  principal  meridian  more  hyperopic  than  the  other ;  or  but 
one  meridian  being  hyperopic,  the  other  emmetropic  or  myopic. 
These  are  defects  of  structure  that  have  been  considered;  the  only 
new  thing  there  is  to  the  astigmatism  is  the  inequality  between  dif- 
ferent meridians.  This  inequality  is  eliminated  by  a  cylindrical  lens 
that,  like  a  bridge,  spans  the  dift'erence  between  the  two,  for  the 
cylinder  is  of  unequal  refraction  in  different  meridians,  and  its  in- 
equalities may  be  made  to  act  as  inequalities  that  are  complementary 
to  those  of  the  eye. 

Being  of  unequal  dioptric  power,  one  principal  meridian  may 
have  2  D.  greater  refraction  than  the  other,  as  if  one  has  +58  D. 
and  the  other  -j-56  D.,  counting  the  refraction  of  both  the  corpea  and 
crystalline  lens,  or  al!  the  positive  refraction  of  the  eye.  Both 
of  the  meridians  have  less  than  the  standard  power  of  +58.50  D. 
assigned  to  the  eye  in  a  previous  statement.  According  to  this,  both 
meridians  are  hyperopic,  one  for  the  value  of  .50  D.  but  the  other 
for  2.50  D.,  making  the  astigmatic  difference  2  D.  Astigmatism  is 
neither  plus  nor  minus,  neither  hyperopic  nor  myopic,  but  these 
terms  apply  separately  to  the  principal  meridians,  and  their  differ- 
ence is  the  astigmatism. 

If  the  more  hyperopic  meridian  is  at  180,  and  is  2.50  D.  hyper- 
opic, while  the  less  hyperopic  meridian  is  at  90,  and  is  but  .50  D. 
hyperopic,  a  +2  D.  cylinder,  ax.  90,  will  make  the  dioptric  power 
of  the  horizontal  meridian  equal  to  that  of  the  vertical,  but  both  will 
still  be  .50  D.  hyperopic.  On  the  other  hand,  a  — 2  cyl.  ax.  180,  will 
cut  the  dioptric  power  of  the  vertical  meridian  down  to  that  of  the 
horizontal,  and  both  will  then  be  2.50  D.  hyperopic.  The  correction 
of  the  eye  is  therefore  as  follows : 

1.  -f-  .50  sph.  C  -|-  2  cyl.  ax.    90.  or 

2.  +2.50  sph.   C  — 2  cyl.  ax.  180. 

But  our  problem  is  not  "What  lens  value  will  correct  the  eye?", 
but,  without  a  lens  correction,  "What  will  be  the  effect  of  the  defect 


MUSCLES   OF  THE   EYE  59 

upon  the  function  of  accommodation?"  Even  this  depends  upon  the 
selection  or  choice,  by  the  reflex  force,  of  the  meridian  to  accommo- 
date for ;  and  no  one  can  tell  in  such  a  case  what  to  do. 

If  .50  D.  spherical  accommodation  is  exercised,  the  vertical  meri- 
dian will  focus  light  from  test  cards  at  20  ft.  on  the  retina,  and  the 
effect  of  this  will  be  to  sharpen  the  definition  of  all  horizontal  lines 
in  letters  or  astigmatic  chart ;  but  all  other  lines,  especially  vertical 
ones,  will  be  blurred  and  dimmed.  On  the  other  hand,  if  2.50  D.  of 
spherical  accommodation  is  exercised,  the  horizontal  meridian  of  the 
eye  will  focus  light  from  20  ft.  on  the  retina,  and  vertical  lines  will 
become  sharply  defined,  but  at  the  expense  of  all  others,  especially 
the  horizontals.  The  clear  images  are  at  right  angles  to  the  focalized 
meridian,  whether  the  accommodation  or  a  lens  focalizes  it. 

In  such  a  case,  if  the  amplitude  of  accommodation  is  abundant, 
the  eye  is  apt  to  do  that  very  thing.  With  less  accommodative  power 
it  may  turn  to  the  other  meridian,  which  can  be  focused  with  but 
.50  D.  accommodation.  The  higher  accommodation  is  exercised  be- 
cause the  sensory  demands  at  the  motor-nerve  center  are  not  satis- 
fied with  anything  else,  provided  the  function  will  respond  to  the 
higher  demand.  Anything  halfway  or  between  the  two,  would  also 
be  unsatisfactory.  We  have  also  the  natural  visual  preference  for 
vertical  lines,  as  they  always  appear  to  be  vertical  and  parallel ; 
whereas  horizontal  lines  appear  slanted,  when  receding  from  or  ap- 
proaching the  eye. 

The  above  exercise  of  accommodation  is  not  cylindrical,  al- 
though it  is  thought  to  be  by  some  optometrists  who  meet  it  in  the 
fitting  room.  If  the  crystalline  lens  can  be  flexed  cylindrically  by  the 
ciliary  muscles,  it  is  for  a  much  less  amount  than  the  above  2  D.  as 
that  would  involve  a  torodial  curve  that  had  about  22c  more  curva- 
ture in  its  maximum  than  in  its  minimum  meridian,  and  this  would 
make  practically  8  D.  astigmatism  at  the  cornea.  Whether  the  eye 
accommodates  cylindrically  or  not,  and  for  an  amount  great  or  small, 
is  beside  the  question.  It  may  endeavor  to  do  so,  and  then  the 
nervous  eflFects  will  probably  appear,  especially  if  the  astigmatism 
is  slight,  so  that  the  efl^ort  will  be  encouraged  by  the  near  prospect  of 
attaining  it.    This  is  not  answering  the  question,  which  we  prefer  to 


60 


MUSCLES    OF  THE   EYE 


leave  as  a  mooted  one,  as  any  answer  to  it  would  only  lead  to  argu- 
ment, without  any  decision. 


FIGURE  14. 

Bye  with  2  D.  compound  hyperopic  astigmatism  "witli  the  rule"  .50  D.  in 
vertical,  2.50  D.  in  horizontal;  Amplitude  of  accommodation,  6  D. ;  2.50  D. 
accommodation  active,  3.50  D.  accommodation  in  reserve;  H,  focus  at  retina, 
V,  focus  2  D.  forward  of  retina;  Vertical  lines  of  astigmatic  chart  clear.  Hor- 
izontal lines  blurred. 

Hyperopic  Induction 

Hyperopia  and  the  consequent  ciliary  activity,  exercises  an  in- 
ductive influence  on  other  muscular  functions,  or  motor-nerve  stim- 
ulations of  those  muscles,  of  the  eyes.  First,  the  sphincter  muscles 
of  the  iris  tend  to  contract  with  the  contraction  of  the  circular  or  the 
sphincter  muscles  of  the  ciliary.  Therefore  the  uncorrected  hyper- 
opic eye  has  usually  a  small  pupil,  due  to  the  associated  action  of 
these  muscles  that  have  analogous  functions.  But  as  the  two  mus- 
cles are  enervated  by  the  same  nerves,  the  3rd  pair  of  cranial  nerves, 
and  have  a  common  or  nearly  common,  motor  nerve-center,  that 
"upon  the  floor  of  the  fourth  ventricle"  etc.,  that  is  usually  ascribed 
as  the  reason  for  their  co-ordinate  action. 

The  association  of  this  ciliary  action  with  that  of  the  internal 
recti  muscles,  also  innervated  by  the  3d  pair  of  cranial  nerves,  has 
already  been  referred  to.  This  influence  is  so  strong  that  chil- 
dren who  have  not  fully  acquired  the  "sense  of  fusion"  necessary 
to  due  convergence  of  the  eyes  for  near  vision,  are  apt  to  over-con- 


MUSCLES   OF   THE    EYE  61 

verge  the  eyes,  or  to  converge  them  for  distant  vision,  causing  di- 
plopia initially,  but  leading  soon  to  the  "repression"  of  visual  regis- 
tration in  one  of  the  eyes  to  avoid  that  result.  Unless  the  hyperopia 
is  soon  corrected  the  deviating  eye  becomes  confirmed  in  its  inward 
turning,  and  the  eyes  are  permanently  crossed.  But  the  repression 
of  vision  in  the  deviating  eye  also  soon  results  in  its  impairment  of 
visual  sensitiveness,  or  to  "amblyopia  ex-anopsia"  or  dimness  of 
vision  due  to  the  non-use  of  the  eye  as  a  visual  organ. 

It  is  thus  found  that  the  ciliary  activity  in  hyperopia  makes  man- 
ifest the  close  association  between  the  ciliary  and  iris  sphincters,  and 
the  converging  extrinsic  muscles,  all  of  which  are  innervated  by  the 
same  pair  of  cranial  nerves,  but  by  different  branches  of  them.  If 
the  superior  and  inferior  recti  are  similarly  influenced,  they  coun- 
teract each  other,  and  the  effect  upon  the  inferior  oblique  is  negli- 
gible. This  association  has,  therefore,  an  anatomical  and  physio- 
logical explanation  that  is  more  direct  than  the  association  of  the 
radial  muscles  of  the  ciliary  with  those  of  the  iris  and  with  the  ex- 
ternal recti  muscles,  which  led  to  the  use  of  the  word  "induction"  to 
represent  their  mutual  associate  relations.  But,  notwithstanding  the 
lack  of  a  direct  nervous  connection,  at  least  of  a  discovered  one,  the 
association  pertains  to  the  latter  quite  as  evidently,  though  not  so  pro- 
nouncedly, as  to  the  former.  This  takes  the  question  to  the  physiol- 
ogist. 

These  associations  are  of  course  reciprocal.  That  is,  if  ciliary 
action  of  either  kind  influences  other  optical  functions  that  are  exer- 
cised by  muscles  through  the  stimulation  of  motor  nerves,  the  exer- 
cise of  those  other  muscular  functions  will  influence  ciliary  action. 
The  entire  ocular  group  of  muscles  are  thus  members  of  the  same 
family  and  co-operate  or  co-ordinate  for  visual  purposes. 


62  MUSCLES  OF  THE   EYE 

CHAPTER  V. 

Vision 

Vision  is  neither  a  structural  nor  muscular  function,  nor  is  it 
a  faculty  of  the  brain;  for  faculties,  such  as  reason,  memory  and 
the  imagination,  pertain  to  the  mind,  and  there  are  no  sensory  chan- 
nels of  communication  between  it  and  the  outer  world.  It  classes  as 
a  special  sense,  and  is  a  modification  of  the  sense  of  feeling  or  touch. 

It  is  obvious  that  the  eye,  both  structurally  and  functionally,  is 
designed  to  provide  the  sense  of  vision  with  the  initial  sensory  touch 
that  is  required  to  set  in  operation  the  further  sensory  activities  upon 
which  vision  depends.  This  initial  touch  is  in  the  form  of  light- 
created  images  at  the  retina  of  the  eye;  and  they  are  placed  there 
by  the  eye,  acting  as  an  anatomical  and  physiological  camera,  resem- 
bling essentially  the  artificial  camera  that  images  objects  by  the  same 
means  on  a  ground-glass  screen. 

It  is  not  in  the  formation  of  these  images,  nor  their  imposition 
on  the  retina,  that  we  have  vision.  So  far  as  light,  refraction  and 
images  are  concerned,  all  that  is  ended  at  the  retina.  The  retina,  op- 
tically considered,  serves  the  same  purpose  as  the  ground-glass 
screen  of  the  camera,  and  no  better.  It  is  only  the  bulletin  board  on 
which  these  images  or  pictures  are  drawn.  It  is  what  follows  this 
imposition  of  the  images  upon  it,  the  sensory  field  on  which  they  are 
imposed,  the  character  of  the  spot  that  is  touched  by  them  that  leads 
on  to  the  awakening  of  the  visual  sense  to  them.  Like  the  chemically 
sensitized  plate  that  records  the  camera  pictures,  there  are,  beyond 
the  retina,  deeper-seated  sensory  organisms  that  respond  to  and  reg- 
ister these  retinal  impressions,  and  these  are  essential  to  visually 
feeling  what  the  retina  displays. 

The  Retina 

Before  considering  the  deeper-seated  sensory  elements,  the 
retina,  as  the  initial  sensory  field,  must  be  given  first  attention.  It  is  a 
membrane  having  a  vast  number  of  sensory  nerve-endings,  imbedded 
in  a  pigment  substance  that  chemically  reacts  to  light,  causing  atomic 
explosions  at  all  points  where  light  touches  it.  The  resulting  agita- 
tion of  the  nerve-endings  (rods  and  cones)  is  communicated  to  the 


MUSCLES   OF  THE   EYE  63 

brain  along  a  sensory  tract,  beginning  with  the  retina  and  ending  at 
the  brain.  At  the  optic  nerve-head  or  disc,  the  retinal  nerves,  after 
undergoing  important  developments  within  the  layers  of  the  retina, 
are  gathered  into  a  cable  of  sensory  nerves  and  pass  out  of  the  eye 
as  the  optic  nerve,  where  we  must  leave  it  for  the  present  to  consider 
the  retina  itself.  t 

As  a  membrane  the  retina  covers  a  little  more  than  half  of  the 
inner  surface  of  the  eye.  Inwardly  it  is  concave,  and  it  is  from  its 
outward  convex  surface  that  the  rods  and  cones  project  into  the 
pigmented  layer  between  the  retina  and  choroid  coat.  Although  thin 
and  transparent,  the  retina  consists  of  ten  layers,  including  the  pig- 
ment layer  into  which  the  rods  and  cones  extend.  The  sensory  qual- 
ities of  the  organized  layers  is  indicated  by  their  differentiation  of 
light  waves  of  greater  or  less  frequency,  by  which  we  have  color 
vision ;  and  between  convex  and  concave  wave  formations,  by  which 
the  impact  of  the  waves  in  diffusion  circles,  locate  the  foci  when  for- 
ward or  back  of  the  retina,  thus  providing  the  "guiding  sensation" 
for  greater  or  less  accommodation. 

On  the  small  area  of  the  retina  is  displayed  the  image  of  every 
object  seen,  and  it  is  of  these  images  that  the  visual  sense  takes  cog- 
nizance. Although  sensitive  to  light  at  all  points,  except  the  optic 
disc,  it  is  specially  sensitive  at  the  small  area  directly  in  line  with 
the  visual  axis,  known  as  the  macula  lutea,  as  this  is  the  field  upon 
which  are  displayed  the  main  features  of  every  image  on  the  retina. 


FIGURE  15. 
Anterior  view  of  the  fundus  or  retina  of  right  eye,  showing  following  de- 
tails:   R,   retina  as  a  whole,  with  arteries  and  veins;  D,  optic  disc  or  nerve- 
head,   to  nasal   side  of   retina;    M,    macula   lutea,    or   "yellow    spot";    F,    fovea 
centralis  in  the  center  of  which  is  located  the  subjective  point  of  fixation  (s.p.f.). 


64  MUSCLES   OF  THE    EYE 

But  near  the  center  of  this  is  a  small  depression,  the  fovea  centraHs, 
that  is  even  more  acutely  sensitive  to  image  impressions.  The  sub- 
jective "point  of  fixation"  (s.p.f.)  is  within  the  very  center  of  this 
minute  area.  That  point  of  an  object  upon  v^^hich  we  center  vision, 
the  objective  "point  of  fixation",  (o.  p.  f.)  is  imaged  at  this  very 
center  of  the  fovea  centralis.  , 

Visual  Acuity 

The  term  acuity,  as  applied  to  vision,  refers  to  the  "keenness" 
of  recognition  of  the  details  of  images  that  are  imposed  upon  the 
retina.  It  is  a  comparative  term,  and  probably  depends  upon  the 
fineness  of  texture  of  the  retina,  or  the  compactness  and  delicacy 
of  structure  of  the  rods  and  cones,  or  the  responsiveness  of  the 
pigmented  layer,  the  "visual  purple",  to  the  action  of  the  light.  It 
has  no  relation  to  the  focalization  of  light  or  to  the  clearness  of  the 
image  upon  the  retina.  The  images  are  impaired  by  imperfect 
focalization,  but  the  acuity  of  vision  is  not  so  impaired.  Acuity  of 
vision  records,  with  fideHty,  the  image  as  it  is,  not  as  it  ought  to  be. 
The  term  "20/20"  is  a  record  of  vision,  but  not  of  acuity  of  vision. 
The  same  eye  may  have  had,  before  correction,  but  20/40  vision, 
although  its  acuity  is  not  changed  by  the  lens. 

Hence,  acuity  of  vision  is  the  capacity  of  the  eye  to  visualize 
objects  in  detail  when  they  are  sharply  imaged  upon  the  retina,  and 
not  what  is  seen  when  there  are  blurred  images  upon  it.  But  even 
then  people  dififer,  although  not  enough  to  prevent  the  acceptance 
of  a  common  standard.  As  we  use  our  eyes  mostly  for  seeing  the 
details  of  drawings  or  printed  matter,  the  standard  is  based  upon 
our  perception  of  letters  or  characters  that  make  a  minimum  visual 
angle  at  the  eye,  and  therefore  have  a  retinal  image  to  correspond  to 
it,  for  the  visual  angle  that  embraces  the  object  also  embraces  the 
image  of  the  object  upon  the  retina.  As  there  must  be  a  contrast 
between  the  image  and  its  background,  there  must  also  be  a  corre- 
sponding contrast  between  the  object  and  its  background.  There- 
fore black  letter  or  figures  on  a  white  background  are  the  accepted 
requirements. 

To  meet  the  accepted  standard  of  size  required  for  clear  visual 
delineation,  the  object  size  must  be  sufficient,  for  its  distance  from  the 


MUSCLES   OF  THE   EYE  65 

eye,  to  make  the  minimum  angle  of  vision  equal  to  i',  or  1/60  of  a 
degree.  In  a  "block  letter"  whose  segments  are  in  the  ratio  of  1  to 
5  to  the  whole  letter,  the  entire  letter  must  fill  a  visual  angle  of  5',  or 
1/12  of  a  degree.  To  recognize,  visually,  letters  of  this  size,  the 
smaller  segments  must  be  seen,  so  that  recognition  of  the  letter  in- 
volves seeing  its  segments.  The  easiest  way  to  calculate  the  size  of  a 
letter  for  any  fixed  distance  is  on  the  basis  of  the  tangent  value  of  a 
5'  angle,  which  is  .00145.  This  is  the  ratio  of  a  full  dimension  of  the 
letter  to  its  distance  from  the  eye.  Hence,  for  a  distance  of  6  meters 
or  6,000  millimeters,  it  is  .00145  of  6,000  mm.  =  8.7  mm.,  or  for  4 
meters  it  is  .00145  of  4,000  =  5.8  mm. 

As  6  meters  is  practically  a  distance  of  20  ft.  letters  of  the  above 
dimensions,  8.7  mm.  tall  or  wide,  or  both,  are  designated,  in  the  line 
of  letters  of  that  size,  line-20.  Hence,  if  the  letters  of  the  line  can 
be  read  off  at  20  ft.,  vision  is  said  to  be  20/20,  or  normal,  according 
to  the  standard.  In  a  letter  chart  letters  are  made  of  normal  size  for 
different  distances,  as  for  30  ft.,  40  ft.,  60  ft.,  100  ft.,  etc.,  and  these 
lines  of  letters  are  termed  line-30,  line-40,  line-60,  line- 100  etc.  When 
a  patient  can  only  see  the  letters  of  line-60  at  a  distance  of  20  ft., 
vision  is  designated  at  20/60 ;  but  this  is  not  visual  acuity,  as  many 
term  it,  but  merely  "vision",  although  the  eye  may  not  be  able  to  see 
better  with  a  lens  correction,  for  that  question  has  not  been  deter- 


FIGURE  16. 
Foveal  vision  of  the  crescent  of  the  new  moon,  the  lower  horn  of  which  Is 
the  objective  point  of  fixation.     This  places  the  imagre  of  the  other  horn  near 
the  border  of  the  fovea. 


60  MUSCLES   OF   THK    EYE 

mined.     Tlie  acuity  of  vision  is  as  good  as  it  can  be  made  with  a 
lens  correction. 

Fovcal  Vision 

We  gauge  visual  acuity  by  what  the  eye  can  see  at  its  most  sen- 
sitive area,  the  fovea  centraHs,  which  is  said  to  be  supplied  with  cones 
only  as  nerve  endings,  or  to  be  without  the  less  delicately  sensitive 
rod.s.  To  give  an  idea  of  what  would  be  embraced  in  a  foveal  image, 
we  may  refer  to  a  statement  of  Tscherning,  to-wit:  that  when  we 
look  at  the  crescent  of  the  new  moon,  and  fix  one  horn  of  it,  thereby 
placing  its  image  at  the  "subjective"  point  of  fixation  at  the  center 
of  the  fovea,  the  image  of  the  other  horn  is  also  on  the  fovea,  but 
nearer  its  margin.  As  the  diameter  of  the  moon  is  2,160  miles,  and 
its  average  distance  from  the  earth  may  be  taken  as  240,000  miles, 
the  tangent  value  of  its  angle  of  vision  at  the  eye  is  the  former 
divided  by  the  latter,  or  .009,  which  represents  an  angle  of  31'. 

Allowing  3'  extra  for  the  uncovered  margin,  the  angular  space 
separating  the  center  of  the  fovea  from  its  margin  is  34',  and  this  is 
but  half  of  it,  and  therefore  the  whole  is  approximately  68',  whose 
tangent  value  is  almost  exactly  .02.  With  this  ratio  as  a  basis  we 
may  calculate  the  size  of  any  object  whose  image  would  be  embraced 
upon  the  fovea,  if  we  know  its  distance  from  the  eye;  or  the  dis- 
tance at  which  an  object  of  a  given  size  would  have  to  be  to  have 
its  image  embraced  by  the  fovea.  On  this  basis,  taking  17  mm.  as 
the  focal-length  of  the  eye,  the  diameter  of  the  fovea  is  .02  of  17  mm. 
or  .34  mm,  =  practically  1/3  mm.  or  radius  of  1/6  mm.  In  the 
Snellen  test  cards,  at  20  ft,,  or  6,000  mm.,  a  circular  space  of  the 
diameter  of  ,02  of  6,000  =120  mm.,  would  be  included  in  it.  In  the 
supplementary  figure  we  show  what  letters  of  line-20  would  fall  upon 
it  when  the  letter  in  the  center  of  line-20  is  fixed.  We  see  only  that 
one  letter  by  direct  vision ;  all  of  the  others  are  seen  by  indirect 
vision.  It  is  the  one  letter  we  are  concentrating  visual  attention 
upon. 

If,  on  the  street,  we  look  at  objects  half  a  city  block  away,  (660 
ft,  for  a  full  block)  the  images  included  in  the  foveal  area  would  be 
,02  of  330  ft.,  or  6.6  ft.  Therefore  a  six-foot  police  officer  at  that 
distance  might  be  imaged  upon  the  fovea,  if  we  directed  attention  to 
a  silver  buckle  on  his  belt.     But  we  could  also  see  many  details  of 


MUSCLES   OF   THE    EYE 


67 


liis  uniform,  for  there  is  quite  a  difference  between  a  .02  and  a 
.00145  'ingl6>  the  latter  being  the  tangent  of  a  5'  angle,  the  former  of 
a  68'  angle.  For  the  given  distance,  the  5'  angle  would  embrace  prac- 
tically sH  inches.  Therefore  the  five-pointed  star  on  his  uniform 
if  of  this  size,  would  be  seen  in  detail  by  one  with  normal  vision. 
This  would  be  the  same  as  looking  at  objects  through  an  aperture  in 
an  opaque  disc.  At  the  reading  distance  of  1/3  meter  or  13".  the 
aperture  would  require  to  be  .26  of  an  inch,  practically  34  inch,  or 
6  mm.  in  diameter.  Hence,  in  reading  at  the  reading  distance,  the 
latter  is  the  extent  of  the  images  that  are  depicted  on  the  fovea  cen- 
tralis at  one  time. 

Fixation 
In  reading  the  test  type  at  twenty  feet,  we  tix  but  one  letter  at 
a  time  out  of  line-20.    The  remaining  letters  of  the  line,  if  the  fixed 
letter  is  the  central  one,  may  be  upon  the  fovea.    Our  diagram  rep- 


FIGURE  17. 
Foveal  vision  of  the  portion  of  a  Snellen  test  card,  as  seen  at  6  meters,  when 
the  central  letter  "Z,"  or  its  middle  segment,  is  the  objective  point  of  binocular 
fixation  reduced  not  quite  one-half. 


68  MUSCLES   OF  THE   EYE 

resents  the  letters  as  they  visually  appear,  not  the  foveal  image  of 
them,  for  that  is  an  inverted  image,  and  the  same  is  true  of  the 
crescent  of  the  new  moon  and  other  figures.  We  have  direct  vision 
of  but  a  small  area  of  any  object,  but  it  is  something  more  than  a 
mathematical  point,  which  is  without  area.  We  may  say  that  the 
standard  minimum  "physical"  point  we  can  fix  (either  with  one  or 
both  eyes)  is  the  i'  minimum  angle,  such  as  the  middle  segment  of 
the  capital  letter  "E"  in  line-20  at  20  ft.  When  vision  is  below  nor- 
mal, as  20/40,  it  may  be  taken  as  the  corresponding  segment  of  a 
letter  from  line-40,  for  one  cannot  have  fixation  of  what  he  cannot 
see,  either  monocularly  or  binocularly.  Fixation  involves  three  essen- 
tials, to-wit : 

1.  Directing  the  visual  axis  to  the  objective  point 
fixed,  so  that  its  image  will  fall  upon  the  subjec- 
tive point  of  fixation,  at  the  center  of  the  fovea. 

2.  Engaging  visual  attention  upon  it,  making  the 
visual  sense  alert  to  the  object  being  visualized, 
rather  than  points  nearer  or  farther  on  the  same 
axis. 

3.  Accommodation  of  the  correct  amount  to  make  the 
image,  whatever  the  distance  of  the  object,  as  per- 
fectly defined  as  possible. 

The  first  two  items  may  be  considered  voluntary ;  but  the  third 
is  involuntary ;  but  by  doing  the  first  two  voluntary  things,  the  third 
will  naturally  co-operate  with  them  by  reflex  action.  We  may  direct 
the  visual  axes  where  we  will,  and  engage  visual  attention  upon  what 
we  choose,  be  it  far  or  near.  But,  having  done  so,  the  accommoda- 
tion does  what  is  required  of  it  to  the  best  of  its  ability.  For  binoc- 
ular fixation,  something  more  is  required. 

Visual  Axis 

The  visual  axis  is  a  line  connecting  the  subjective  and  objective 
points  of  fixation.  Objectively  considered,  the  "point  of  fixation" 
is  in  the  object,  and  an  axial  ray  of  light  passes  from  it  to  the  cen- 
ter of  the  fovea  centralis.  It  is  too  near  the  optic  axis  to  suffer  but 
the  slightest  refraction  at  any  surface,  and  is  therefore  a  straight 
line.  Other  rays  of  light  from  the  same  point  suffer  slight  refrac- 
tion,  for  they  are  converged  to  a   focus  at  the  retina,  or  at  the 


MUSCLES   OF  THE   EYE  69 

subjective  point  of  fixation  or  foveal  center.  But,  subjectively  con- 
sidered, the  visual  axis  is  a  line  of  projection  merely,  extending 
from  the  subjective  point  of  fixation  out  into  space,  and  to  the  ob- 
jective point.  It  is  not  light  and  therefore  suffers  no  refraction,  nor 
are  there  any  side  rays  to  be  refracted  although  we  may  have,  the- 
oretically, such  a  pencil  of  rays ;  and  actually  do  have  them  in 
shadow-testing  an  eye. 

Regarded  subjectively,  it  is  the  retinal  image  of  the  object  we  see 
that  is  the  original  manifestation  in  any  visual  sensation.  The  ob- 
ject, visually  regarded,  is  a  projection  of  the  retinal  image.  But  all 
of  our  experiences  in  the  physical  world  teach  us  that,  for  such  a 
sensation,  there  is  an  objective  physical  cause.  The  retinal  image  of 
a  vicious  dog  could  not  bite  us,  nor  the  retinal  image  of  a  speeding 
automobile  run  us  down.  Nevertheless,  when  we  receive  these  ret- 
inal sensations  or  warnings,  we  endeavor  to  get  out  of  danger,  for 
we  know  that  any  on-looker  might  otherwise  have  the  visual  sensa- 
tion of  a  dog  biting  a  man  or  an  automobile  running  him  down  and 
injuring  him.  We  therefore  dismiss  this  vague  theory  of  subjective 
originality  and  consider  the  real  things  by  which  we  are  so  evidently 
surrounded  and  the  light  that  makes  images  on  the  retina  as  the  basis 
for  visual  sensations. 

When  we  look  through  a  wire  netting  or  screen  at  objects 
beyond  it,  by  fixing  a  particular  object  we  direct  the  visual  axis  to  it. 
the  visual  sense  is  made  alert  to  it,  and  the  accommodation  focalizes 
the  light  from  it  and  images  the  object  upon  the  retina.  The  image 
is  an  inversion  of  the  object,  but  the  object  is  also  an  inversion  of 
the  image.  The  latter  is  objectively  the  real  thing.  But  the  image 
is  also  real.    Which  of  the  two  is  inverted  depends  upon  our  point 


FIGURE  18. 
Representing  the  "angle  of  vi.sion,"  visual  axis,  points  of  fixation,  and  other 
details;   O,   objective  point  of  fixation    (o.p.f.);   P,   subjective  point  of  fixation 
(s.p.f.);    OF,    visual   axis;    ONH    or  FNK,    objective   and   subjective    "angle   of 
vision";  N,  jMjsterlor  nodal  point  of  eye. 


70  MUSCLES   OF  THE   EYE 

of  view.  We  prefer  to  regard  the  whole  matter  from  the  physical 
standpoint,  that  it  is  the  object  that  is  real,  and  that  the  image  is  but 
an  inverted  picture  of  it  in  miniature.  While  we  are  visualizing  an 
object  through  the  screen,  we  have  a  shadowy  definition  of  the  wire 
netting  imaged  upon  the  retina.  The  visual  axis  passes  through  it 
to  the  object  on  which  attention  is  fixed ;  but  our  visual  sense  is  not 
alert  to  it,  and  we  are  not  therefore  accommodating  for  it.  But,  we 
may,  in  a  moment,  fix  attention  upon  the  wire,  and  withdraw  it  from 
the  object  beyond  it.  Then,  although  there  is  no  change  in  the  direc- 
tion of  the  visual  axis,  the  accommodation  at  once  focalizes  the  light 
from  the  wire  netting  at  the  retina,  and  our  vision  of  the  object 
beyond  it  becomes  shadowy  and  indistinct,  although  we  may  still 
see  it  by  indirect  vision. 

Indirect  Vision 

Direct  vision  is  our  means  of  scrutinizing  the  details  of  an 
object,  and  we  do  that  by  fixing  successively  its  different  objective 
points  or  features,  and  in  such  rapid  succession  that  we  visually 
grasp  the  whole,  or  an  area  much  wider  than  the  fixation  point.  But 
the  initiative  for  turning  visual  attention  from  one  point  to  another, 
and  the  visual  axis  from  one  direction  to  another,  is  the  indirect 
vision  of  those  other  points  of  interest  embraced  within  our  entire 
field,  and  to  which  we  may  presently  turn  visual  attention  for  greater 
details,  or  for  a  more  detailed  inspection  of  them.  This  applies  not 
only  to  lateral  points,  to  points  in  all  directions  from  the  point  of 
fixation,  but  to  points  farther  away  or  nearer  to  the  eye.  Vision  is 
thus  kept  perpetually  on  guard  and  indirect  vision  plays  a  not  less  im- 
portant part  than  direct. 

The  images  of  objects  being  seen  by  indirect  vision  are  placed 
on  the  retina  by  pencils  of  light  that  pursue  a  course  oblique  to  that 
of  the  light  from  the  point  of  fixation.  But  these  pencils  have  an 
axillary  ray  or  one  whose  course,  after  refraction  by  the  dioptric 
media,  is  parallel  with  its  incident  course,  and  it  is  a  secondary  axis 
unless  it  happens  to  be  that  ray  that  is  known  as  the  optic  axis,  and 
which  pursues  its  course  to  the  retina  in  a  mathematically  straight 
line.  But,  it  reaches  the  retina  slightly  to  the  nasal  side  of  the  sub- 
jective point  of  fixation,  at  the  center  of  the  fovea.    Or  we  may  say. 


MUSCLES   OF  THE   EYE  71 

that  the  visual  axis  is  slightly  oblique  to  the  optic  axis,  as  the  point 
of  fixation  at  the  retina  is  slightly  to  the  temporal  side  of  the  posterior 
pole  of  the  eye.  It  is  this  position  of  the  fovea  and  of  the  macula 
that  provides  the  theorist  with  those  intangible  angles,  the  angle  a 
or  alpha,  and  angle  gavnna,  the  latter  being  purely  hypothetical. 

Cardinal  Points   •  • 

It  is  these  angles  also,  as  well  as  the  proportion  of  refraction  of 
the  different  dioptric  surfaces  of  the  eye,  due  to  differences  in  curva- 
ture and  index,  that  locate,  along  the  optic  axis  those  points  that  are 
known  as  the  cardinal  points  of  the  eye.    They  are  respectively : 

1.  The  Anterior  Principal  Point. 

2.  The  Posterior  Principal  Point. 

3.  The  Anterior  Nodal  Point. 

4.  The  Posterior  Nodal  Point. 

5.  The  Anterior  Principal  Focus. 

6.  The  Posterior  Principal  Focus. 

As  these  technics  have  really  nothing  to  do  with  the  visual 
axis,  and  it  is  the  line  of  greatest  interest  in  the  study  of  the  muscu- 
lar functions,  we  can  leave  them  to  the  mathematician  who  loves  to 
delve  in  abstractions  rather  than  direct  his  attention  to  the  matters 
of  real  concern,  and  that  have  a  practical  value  to  those  who  may 
read  this  book,  which  we  hope  to  make  useful  rather  than  ornamental. 

The  Sensory  Tract 

At  the  optic  nerve  head  or  disc,  which  is  about  i/io  diameter 
of  the  eye  to  the  nasal  side  of  the  posterior  pole  of  the  eye,  the  sen- 
sory nerves  from  the  retina  are  gathered  into  a  single  cable,  and  pass 
out  of  the  eye  as  the  optic  nerve.  But,  in  the  optic  nerve,  they  do 
not  lose  their  identity,  but  remain  distinct,  the  same  as  the  wires  in 
a  telephone  cable.  The  optic  nerve  passes  back  through  the  orbit  to 
the  optic  foramen,  and  through  that  aperture  into  the  skull.  It  then 
extends  to  the  optic  commissure  or  chiasm,  at  which  it  undergoes  an 
important  division  and  reclassification. 

First,  there  is  a  division  of  the  optic  nerve  into  two  branches, 
one  of  them  embracing  all  of  the  sensory  nerves  from  the  right  half 


172  MUSCLES  OF  THE   EYE 

of  the  retina,  and  the  other  all  of  the  sensory  nerves  from  the  left 
half  of  it.  As  there  are  two  optic  nerves,  one  from  each  eye,  branch- 
ing in  this  manner,  to  get  those  nerves  together  that  are  from  corre- 
sponding areas  of  the  two  retinae,  one  division  of  each  nerve  must 
cross  to  the  other  side.  Hence,  that  division  of  the  right  optic  nerve 
that  embraces  nerves  from  the  left  half  of  the  retina  of  the  right 
eye,  and  that  half  of  the  left  optic  nerve  that  embraces  nerves  from 
the  right  half  of  the  retina  of  the  left  eye,  cross  over  at  the  chiasm, 
and  join  the  other  half  of  the  nerve  cable  remaining  on  the  same  side 
as  its  origin  at  the  retina.  The  two  reclassified  cables  then  proceed 
to  the  right  and  left  hemispheres  respectively,  of  the  brain,  as  the 
right  and  left  optic  tracts. 

The  sensory  effect  of  this  classification  is  to  give  each  eye  sen- 
sory connection  with  both  hemispheres  of  the  brain,  for  the  right 
half  of  the  right  optic  nerve  extends  to  the  right  hemisphere  and 
the  left  half  to  the  left  hemisphere ;  while  the  right  half  of  the  left 
optic  nerve  extends  to  the  right  hemisphere,  and  the  left  half  to  the 
left  hemisphere.  If  we  may  speak  of  the  brain  as  "seeing"  the 
images  depicted  on  the  retinas,  the  right  brain  "sees"  what  is  de- 
picted on  the  right  half  of  the  retinas  of  each  or  both  eyes,  while  the 
left  brain  "sees"  what  is  depicted  upon  the  left  halves  of  the  two 
retinas.  Hence  there  is  two-brain  vision  of  the  entire  image  upon 
each  retina ;  or  the  entire  images,  single  at  the  retina  of  each  eye,  are 
duplicated  in  brain  vision  of  them.  Naturally  we  might  expect  to 
see  doubly,  or  have  double  vision,  and  sometimes  we  do,  greatly  to 
our  annoyance  and  distress.  This,  even,  is  binocular  vision,  but  not 
the  most  acceptable  kind. 

Binocular  Single  Vision 

The  right  eye  has  a  complete  image  of  the  objects  before  it,  and 
the  sensory  reaction  of  this  image  is  conveyed  to  both  brains,  the 
right  to  the  right  brain,  left  to  the  left  brain.  But  the  two  half 
pictures  are  dove-tailed  together  with  such  nicety  that  you  do  not  sus- 
pect it,  as  the  two  brains  are  most  efficiently  connected  by  a  sensory 
union.  The  entire  image  on  the  left  retina  is  visualized  in  the  same 
manner.  But,  since  at  the  chiasm,  all  the  sensory  nerves  from  corre- 
sponding sides  of  the  two  eyes,  right  for  right  and  left  for  left,  are 


MUSCLES   OF  THE   EYE  73 

joined  together  in  the  optic  tracts,  they  go  together  as  one  sensory 
registration  to  the  brain,  provided  every  rod  or  cone  in  the  retina  of 
the  right  eye,  or  its  sensory  nerve  extension  to  the  chiasm,  finds 
there,  or  at  some  point  farther  on,  the  sensory  nerve  from  a  corre- 
sponding rod  or  cone  of  the  retina  of  the  left  eye.  It  is  the  crossing 
over  or  decussation  of  nerves  at  the  chiasm  that  has  made  this  find- 
ing, and  their  joining  before  the  sensory  report  is  turned  in  at  either 
brain,  possible.  But  if  there  is  no  such  joining  made,  two  sensory 
reports  are  registered,  and  vision  is  unavoidably  doubled.  Hence  the 
necessity  of  optically  placing  the  two  images  on  corresponding  points 
of  the  two  retinas. 


FIGURE  19. 

Double  sensory  visual  tract  from  retinae  to  brain:  1,  right  hemisphere  of 
brain;  2,  left  hemisphere  of  brain;  3,  right  optic  tract;  4,  left  optic  tract;  5, 
chiasm  or  optic  commissure;  6,  right  optic  nerve;  7,  left  optic  nerve;  8,  right 
optic  disc,  9,  left  optic  disc;  10,  right  half  of  right  retina;  11,  right  half  of  left 
retina;  12,  left  tialf  of  right  retina;  13,  left  half  of  left  retina;  F,  fovea  of  right 
eye;  F*,  fovea  of  left  eye. 

Such  sensory  union  of  the  two  retinal  images  is  termed  fitsion 
of  the  images ;  and  fusion  of  the  images  gives  us  single  vision  of  a 
single  object,  notwithstanding  its  duplication  on  the  retinas  of  the 
two  eyes.  But  the  two  images  must  be  so  placed  upon  the  two  ret- 
inas as  to  fulfill  the  requirements  for  a  single  sensory  report  or  reg- 


74  MUSCLES  OF  THE   EYE 

istration  of  every  point  in  the  object  that  is  thus  duplicated  in  the 
retinal  images.  It  is  at  the  brain  that  these  sensory  reports  are  in- 
tercepted. We  must  therefore  be  provided  with  the  means  of  either 
moving  the  images  to  corresponding  positions  on  the  two  retinas ;  or 
of  moving  the  retinas  to  corresponding  positions  of  the  two  images. 
For  the  first,  our  only  means  of  getting  the  images  together  when 
they  are  not  normally  so,  is  to  divert  or  alter  the  course  of  light  from 
the  object,  for  one  or  both  eyes,  which  would  require  artificial  agents. 
But,  for  the  second,  our  means  is  by  rotating  one  or  both  eyes  in 
their  orbits  to  corresponding  relative  positions,  and  this  is  where  the 
extrinsic  muscles  of  the  eyes  function  very  importantly. 

The  union  of  the  two  images  into  one,  or  fusion,  cannot  take 
place  except  at  the  visual  center  of  the  brain,  but  the  sensory  basis 
for  it  may  be  regarded  as  occurring  at  the  chiasm,  where,  if  the  corre- 
sponding sensory  nerves  are  not  actually  united,  they  are  effectively 
joined  at  that  point,  for  the  classification  of  nerves  that  puts  them 
into  a  single  optic  and  sensory  tract  does  occur  at  the  chiasm.  No 
one  can  say  at  what  exact  point  these  sensory  things  are  transmuted 
into  vision,  which  is  a  psychic  rather  than  sensory  manifestation,  a 
miracle  rather  than  a  natural  phenomenon.  To  the  brain  is  as  far  as 
we  can  go  in  the  investigation  of  its  causes  and  effects,  and  so  far, 
we  have  dealt  only  with  the  sensory  effects,  and  avoided  the  term 
"vision"  as  far  as  it  could  be  avoided  and  express  the  thought  we 
had  in  mind. 

Normal  Violation  of  Rule 

The  above  "rule  of  correspondence"  of  the  images  is  a  visual 
one,  a  visual  requirement  for  the  fusion  of  the  images  and  for  binoc- 
ular single  vision.  But  here  a  law  of  optics  interposes  and  blocks 
this  visual  rule,  and  makes  the  strict  fulfillment  of  it  impossible, 
except  for  a  single  point  of  the  object,  the  o.  p.  f .  or  objective  point 
of  fixation.  The  two  eyes  receive  light  from  any  object  before  them 
from  slightly  different  directions,  so  that  the  two  retinal  images  can- 
not possibly  fall  upon  exactly  identical  or  corresponding  points  of 
the  two  retinas  over  their  entire  area.  In  looking  at  the  upright 
trunk  of  a  tree  that  is  clearly  seen  binocularly  at  a  distance  of  20 
to  30  feet,  the  right  eye  will  see  farther  around  it  to  the  right  than 


MUSCLES   OF  THE   EYE 


75 


the  left  eye ;  while  the  left  eye  sees  farther  around  it  to  the  left  than 
the  right  eye.  The  retinal  images  differ  from  each  other  in  the  same 
way,  and  to  the  same  extent,  so  that  this  is  an  optical  block  to  the 
rule  of  correspondence. 

But  all  of  the  points  of  the  object,  except  the  point  of  fixation, 
are  seen  by  indirect  vision,  and  indirect  vision  is,  as  we  have  seen 
of  a  shadowy  and  less  exacting  character  than  direct  vision.  As  the 
object  seen  is  visualized  as  one  object,  notwithstanding  this  obvious 
and  immutable  optical  law  or  principle,  it  is  manifest  that  the  rule 
of  correspondence  exacts  no  such  impossibility,  but  is  satisfied  with 


FIGURE  20. 
Binociolar  fixation  of  the  point  O,  in  the  line  AOB.    The  image  of  A.  B  and  O 
are  at  a  and  a',  b  and  b',  o.  and  o'  respectively  in  the  right  and  left  eyes,  or 
upon  the  retinae. 


76  MUSCLES   OF   THE   EYE 

exact  correspondence  for  the  s.  p.  f.'s,  or  subjective  points  of  fixa- 
tion, and  the  visual  sense  will  fuse  the  images  if  they  stand  in  this 
relative  position,  and  overlook  or  disregard  violations  of  it  in  fields 
of  indirect  vision.  Therefore,  points  indirectly  seen  do  not  fulfill 
the  rule  of  correspondence,  nor  can  they,  from  the  nature  of  things, 
be  made  to  do  so  without  themselves  becoming  the  s.  p.  f.'s,  or 
vision  being  turned  to  them  as  the  new  points  of  fixation,  and  tak- 
ing the  place  of  the  o.  p.  f .  previously  fixed. 

But  this  "permit"  we  may  call  it,  to  violate  the  rule  of  corre- 
spondence, is  not  a  license  to  do  so  to  any  degree  except  only  for  the 
amount  that  the  optical  law  imposes  upon  it,  and  it  may  therefore  be 
termed  as  above,  a  normal  violation  of  the  rule. 

Stereoscopic  Vision 

It  is  often  said  that  "there  is  no  great  loss  without  some  small 
gain",  but  this  is  an  illustration  so  often  furnished  by  optics  and 
vision,  of  a  very  great  gain  at  a  very  small  loss,  if  it  is  a  loss  at  all, 
for  it  is  the  violation  of  the  rule  of  correspondence  at  all  points,  ex- 
cept the  one  point  of  fixation,  that  provides  us  with  stereoscopic 
vision,  or  makes  it  possible  to  visualize  distance,  as  well  as  the  form 
and  color  of  objects.  That  is,  we  are  not  only  able  to  see  them,  in 
due  form  and  color,  but  we  can  definitely  see  their  relative  positions 
and  our  own  position  relative  to  them.  This  is  what  is  termed 
orientation.  But  the  visual  setting  of  a  vast  number  of  objects  in 
due  position,  relative  to  each  other,  as  they  are  arranged  in  any 
natural  scene,  we  call  the  perspective  of  the  picture,  and  stereoscopy 
of  vision  contributes  mightily  to  these  visual  qualities. 

The  degree  of  violation  of  the  principle  of  correspondence,  by 
the  optical  law,  depends  upon  the  distance  of  the  indirectly  seen  ob- 
jective point  from  the  point  of  fixation,  or  o.  p.  f .  When  we  binoc- 
ularly  fix  any  objective  point,  the  visual  axis  of  each  eye  extends 
from  the  s.  p.  f .  of  each  eye  to  the  one  o.  p.  f . ;  or  light  from  the  one 
o.  p.  f .  proceeds  directly  to  each  s.  p.  f .  of  the  right  and  left  eyes,  as 
shown  in  an  accompanying  figure  or  diagram.  While  the  two  eyes 
are  thus  fixing  this  one  objective  point,  all  other  points  are  seen  by 
indirect  vision.  The  two  s.  p.  f.'s  will  visually  coincide,  but  the  vio- 
lation of  coincidence  at  all  other  points  of  the  retinas  will  depend 


MUSCLES   OF   THE    EYE  11 

upon  their  distance  or  distances  from  the  s.  p.  f.  of  the  eye  or  retina 
on  which  they  are  displayed  as  images ;  and  this,  for  any  single  ob- 
ject, depends  upon  its  nearness  to  the  eyes.  The  farther  the  object 
is  from  the  eyes,  the  nearer  all  points  of  it  or  of  their  images,  will 
cluster  around  the  s.  p.  f .'s,  for  the  images  will  be  smaller ;  and  the 
nearer  the  object  to  the  eyes,  and  therefore  the  larger  the  images,  the 
more  its  objective  points  will  be  spread  out  in  the  image,  and  the 
greater  their  violation  of  the  rule  of  correspondence,  or  augmenta- 
tion of  space  separating  the  images  of  corresponding  points  of  the 
object. 

We  are  thus  provided  with  the  visual  gauge  for  judging  the 
distance  of  an  object,  or  the  relative  distances  of  two  objects,  or 
whether  the  object  that  is  moving  before  us  is  approaching  or  reced- 
ing from  us,  and  at  what  velocity  or  speed.  It  is  not  an  absolute 
measurement  of  distances,  but  a  visual  one;  but  it  sets  the  array  of 
objects  in  any  natural  scene  before  us  in  perspective,  which  is  often 
a  part  of  its  beauty,  and  sometimes  a  feature  of  its  ugliness,  so  that 
we  may  know  what  to  court  and  what  to  avoid,  in  feasting  our  eyes 
on  the  beauties  and  uglinesses  of  nature.  No  art,  except  an  art  that 
involves  real  vision  as  a  finality,  can  contribute  to  our  pleasure  or 
distress  in  such  matters ;  but  with  that  finality,  there  are  many  ways 
of  enhancing  vision  and  of  recording  and  preserving  what  we  have 
once  seen,  although  never,  except  in  the  case  of  memory  or  the 
imagination,  with  the  fidelity  and  vividness  of  the  original  vision  of 
the  same  objects. 

Our  retinas  are  in  the  nature  of  "moving  picture"  screens, 
recording  in  brief  displays  all  that  is  going  on  around  us.  To  change 
the  view  it  is  only  necessary,  without  moving  our  bodies,  to  change 
the  general  direction  of  vision.  By  a  short  walk  we  can  greatly  add 
to  the  variety  of  the  things  seen.  By  travel  this  variety  is  greatly 
increased,  and  all  we  see  is  seen  binocularly,  stereoscopically,  in 
perspective,  and  both  stationary  and  moving  objects  appear  as  they 
are,  and  even  any  movement  we  may  make  that  shifts  the  scene  is 
recorded  with  the  same  accuracy.  All  of  which  goes  to  show  that 
we  have  with  us  always,  in  binocular  vision,  a  moving  picture  pro- 
gram very  much  superior  in  operation  and  display  to  any  we  pay 
from  lo  cents  to  as  many  dollars  to  see  on  canvas.   Why,  then,  the 


78  MUSCLES    OF   THE    EYE 

craze  for  moving  pictures  ?  It  is  entirely  on  account  of  what  we  see 
at  them.  We  cannot  go  out  and  see  a  man  falling  from  an  airplane, 
an  explosion  at  a  factory,  an  automobile  collision,  or  a  complete 
drama,  with  all  characters  moving  about  as  on  the  stage,  whenever 
we  wish  to.  The  Moving  Picture  Show  provides  us  with  these 
diversions,  and  so  we  go  to  them  and  cheerfully  pay  the  price  of 
admission,  including  the  tax  as  well  as  the  taxi,  to  get  us  to  and 
from  it. 

We  have  other  means  than  stereoscopic  vision  to  judge  the 
distance  of  an  object  by,  as  the  convergence  of  the  eyes  required  to 
fix  it,  and  the  accommodation  of  the  eyes  required  to  focalize  it.  But 
these  are  the  mere  exercise  of  the  muscular  functions  of  the  eyes,  of 
which  we  are  unconscious,  and  there  are  other  factors  than  distance 
to  affect  them.  On  the  other  hand,  stereoscopic  vision  is  a  visual 
manifestation.  A  hyperope  doesn't  know  that  he  accommodates  when 
looking  at  distant  objects;  nor  that  he  accommodates  more  when 
looking  at  near  objects,  nor  that  he  converges  the  eyes  more  or  less 
for  either.  These  things  are  done  involuntarily  and  unconsciously. 
But  the  stereoscopic  effects  of  distance  manifest  themselves  to  his 
vision,  although  he  may  not  be  conscious  of  the  underlying  causes. 
The  scene  displays  itself  before  him  and  he  takes  it  in,  nor  can  he 
rid  himself  of  the  visual  manifestation  of  it,  unless  he  does  so  volun- 
tarily, by  closing  or  obscuring  one  eye.  Even  then,  the  preceding 
binocular  effects  remain  with  him,  as  the  images  do  not  fade  at  once, 
but  linger  in  the  visual  sense. 

Image  Displacements. 

We  tolerate,  without  serious  impairment  of  fusion  or  single 
vision,  many  and  often  quite  serious  differences  of  the  two  retinal 
images,  and  see  the  object  better  with  both  eyes,  one  image  being 
much  clearer  and  better  seen  than  the  other,  than  we  can  see  it  with 
the  better  eye  alone.  These  differences  may  be  due  to  differences 
m  refraction  of  the  eyes,  to  differences  of  visual  acuity  in  them,  to 
differences  of  accommodative  power,  for  these  may  not  disturb  the 
visual  fusion  of  the  s.  p.  f.'s,  and  we  are  accustomed  to  differences 
at  other  points.  But,  a  relative  misplacement  or  displacement  of  the 
two  s.  p.  f.'s,  which  at  once  gives  double  vision,  is  profoundly  dis- 


MUSCLES   OF  THE   EYE  79 

turbing,  disagreeable  and  intolerable.  The  ocular  muscles  at  once 
attempt  to  relieve  this  intolerable  visual  effect,  to  bring  these  two  sub- 
jective points  together  by  rotating  the  eyes  to  a  proper  relative  posi- 
tion. If  they  are  unable  to  do  so,  we  repress  or  suppress  the  sensory 
reception  of  such  image,  fixing  our  visual  attention  on  one  of  them 
alone,  and  allow  the  other,  or  force  it,  to  keep  in  the  background  of 
the  visual  sense. 

The  muscles,  when  so  engaged,  do  not  move  the  images,  but 
bring  the  retinas  to  the  proper  relative  positions.  This  may  re- 
quire a  rotation  of  but  one  of  the  eyes.  But  as  there  is  a  natural  in- 
clination of  the  two  eyes  to  rotate  together  in  the  same  direction,  such 
a  rotation  of  one  of  them,  with  the  other  remaining  stationary,  may 
be  impossible;  or  if  it  is  possible,  the  fixing  eye  must  be  held  to  its 
normal  position  while  the  rotating  eye  is  turned  into  its  position  to 
correspond  with  it,  so  that  the  stationary  eye  is  put  under  as  great 
muscular  tension  to  retain  its  normal  position,  while  the  other  eye 
rotates  to  it,  as  the  rotating  eye  to  be  brought  into  correct  alignment 
with  its  binocular  mate.    Therefore,  such  muscular  tensions  for  the 


FIGURE  21. 
Devices  employed  to  deceive  vision  and  eliminate  stimulus  of  visual  sense 
for  fusion  of  the  Images     D,  double-prism,   the  central  line  showing  position 
of  bases  of  prisms;  M,  Maddox  rod,  single  form,  with  an  oblong  aperture  In  disc 
for  mounting.    Horizontal  rods  make  single  light  into  vertical  streak. 

purpose  of  bringing  about  fusion,  are  always  binocular  in  character. 
They  are  participated  in  by  both  of  the  eyes,  or  by  the  muscles  of 
both,  and  equally  by  the  two  eyes  or  by  their  muscles.  This  fact  is 
as  elementary  or  fundamental  as  the  fact  that  2  and  2  make  4.  They 
are  the  pairs  of  muscles  that  rotate  the  eyes  in  opposite  directions. 

But  the  muscles  may  be  relieved  of  this  action  by  employing 
prisms,  or  a  single  prism  before  one  eye,  to  so  direct  the  course  of 


80  MUSCLES   OF  THE   EYE 

light  as  to  place  the  images  in  the  relative  positions  of  fusion,  thus 
saving  the  muscles  the  strain  of  rotating  the  eyes  as  required.  A 
prism  of  the  correct  value  to  unite  the  two  images  may  stand  before 
either  eye,  but  its  required  direction  of  deviation  for  the  right  eye 
must  be  exactly  opposite  to  its  direction  of  deviation  for  the  left  eye. 
It  moves  the  image  to  vi^here  it  requires  to  stand  to  fuse  vi'ith  the 
image  in  the  other  eye.  A  prism,  since  it  satisfies  the  demand  for  a 
fusion  position,  is  a  binocular  optical  agent.  In  any  means  that  may 
be  employed  to  fuse  images,  muscular  or  prismatic,  even  though  the 
muscle  rotates  but  one  of  the  eyes  or  the  prism  moves  the  image  in 
but  one  of  them,  w^e  are  optically  dealing  with  both  of  the  eyes,  and 
one  as  much  as  the  other. 

Visual  Deception 

To  cause  a  pair  of  eyes  to  reveal  their  true  muscular  condition 
it  is  necessary  to  eliminate,  in  some  manner,  the  natural  desire  that 
the  two  similar  images  be  fused  into  one,  and  so  quiet  the  muscular 
stimulation  needed  to  satisfy  the  fusion  sense.  We  do  this  by  em- 
ploying a  device  that  makes  one  of  the  images  dissimilar  in  color, 
form,  position  or  number  from  the  other.  Only  one  of  the  eyes 
needs  to  have  its  image  so  treated.  The  devices  most  used  are  as 
follows : 

1.  The  Single  Prism,  with  any  small  target,  as  a  line  or  letter  on 

a  distant  chart. 

2.  A  Double  Prism,  with  the  same  target  as  above,  or  the  line- 

and-dot  at  any  convenient  distance. 

3.  The  Maddox  Rod,  with  a  small  bright  light  at  20  ft.  as  target. 

A  colored  disc  is  sometimes  used  with  either  of  the  above,  or 
by  itself.  The  Pyramid  device  is  a  development  of  the  2d,  and  the 
Cone  device  is  a  development  of  the  3d.  These  devices,  placed  in  a 
trial  cell,  may  be  rotated  to  any  position  desired. 

The  Single  Prism,  placed  before  one  eye,  so  displaces  the  image 
of  the  target  that  double  vision  is  unavoidable.  We  may  then  ob- 
serve what  relative  positions  the  two  images  have,  and  ascertain 
whether  there  is  a  muscular  factor  operative  tending  to  separate 
them. 


MUSCLES   OF  THE   EYE  81 

The  Double  Prism  produces  diplopia  of  the  eye  over  which  it 
stands.  The  positions  of  these  two  images,  relative  to  the  single  one 
in  the  uncovered  eye,  then  tell  what  the  muscular  tendencies  of  the 
eyes  are,  normal  or  abnormal. 

The  Maddox  Rod  converts  the  small  bright  light  into  a  straight 
lineal  streak  of  light,  and  the  single  hght,  seen  by  the  uncovered  eye 
is  directly  in  it  or  not,  according  to  the  muscular  condition  of  the 
eyes.    Its  direction  from  the  streak  indicates  the  abnormality. 


82  MUSCLES   OF  THE   EYE 

CHAPTER  VI 

Axillary  Control 

Since  the  visual  axis  of  an  eye  extends  from  the  center  of  the 
fovea  centralis  out  through  the  dioptric  media  into  space,  it  is  fixed 
in  position  relative  to  the  eye  ;  but  rotations  of  the  eye  in  its  orbit  give 
it  a  different  direction,  so  that  its  other  extremity  may  be  changed 
as  desired  by  such  rotations.  The  objective  point  on  which  vision  is, 
for  the  moment,  centered,  is  at  its  objective  extremity.  It  is  there- 
fore a  line  extending  from  the  subjective  point  of  fixation,  s.  p.  f.. 
to  the  objective  point  of  fixation,  o.  p.  f .,  and  maintains  that  charac- 
ter for  all  directions. 

In  binocular  vision  there  are  two  subjective  points  of  fixation, 
one  for  the  right  eye  and  one  for  the  left.  But  there  can  be  but  one 
objective  point  of  fixation,  even  with  diplopia  or  double  vision,  as 
"fixation"  involves 

1.  Directing  the  visual  axis  to  the  objective  point. 

2.  Adapting  the  accommodation  to  its  distance,  and 

3.  Visual  alertness  to  the  object  visualized. 

In  diplopia,  visual  attention  may  be  fixed  upon  either  of  the  two 
objects  that  appear.  The  eye  that  fixes  it  sees  it  by  direct  vision,  the 
other  eye  sees  it  by  indirect  vision.  But  we  may  reverse  this  by  giv- 
ing visual  attention  to  the  second  of  the  two  objects  seen.  In  binoc- 
ular single  vision  both  eyes  at  once  meet  the  above  requirements.  The 
two  images  are  then  fused,  at  the  retinas,  at  the  chiasm  or  at  the 
visual  centers  of  the  brain. 

As  we  live  in  a  moving  world,  and  ourselves  move  about  in  it, 
it  is  quite  essential  that  we  have  the  means  of  adjusting  the  positions 
of  the  visual  axes,  both  to  the  different  directions  of  objects  from 
the  eyes,  and  their  different  distances  from  us.  The  first  is  required 
by  each  eye  singly,  the  latter  by  the  two  eyes  as  binocular  organs,  for 
an  objective  point  cannot  be  in  exactly  the  same  direction  from  both 
of  the  eyes,  especially  when  we  are  near  to  it.  For  near  objects  it  is 
necessary  that  the  eyes  be  converged  to  the  one  point  of  fixation.  It 
is  a  physical  as  well  as  a  visual  necessity. 


MUSCLES   OF   THE    EYE  83 

We  are  able  to  change  the  direction  of  a  visual  axis  by  changing 
the  position  of  the  head,  or  by  tilting  it  up,  down,  to  right  and  to  left ; 
we  can  also  move  the  body  so  as  to  change  the  direction  of  the  visual 
axes.  But  these  movements  would  be  too  slow  and  uncertain  for  us, 
and  we  cannot  converge  the  eyes  by  either  a  head  or  body  movement, 
as  required  for  near  vision,  nor  to  prevent  diplopia  when  the  eyes  or 
their  axes  are  not  in  true  alignment  for  binocular  single  vision.  Orbi- 
tal rotation  of  the  eyes  provide  this  scope  of  movement. 

Mountings  and  Muscles 

Each  eye  is  enclosed  in  an  (i]:»en-mouthed  membranous  sac,  the 
Capsule  of  Tenon,  in  which  it  may  rotate  freely  within  certain  limits, 
but  is  restrained  from  turning  too  far  by  check  ligaments  that  cross 
from  it  to  the  inner  surface  of  the  capsule.  The  cornea  and  forward 
part  of  the  sclera  are  exposed  at  the  anterior  opening  of  the  capsule 
to  air  and  to  incident  light.  The  eye.  as  thus  enclosed,  is  set  or 
mounted  in  the  socket,  and  protected  anteriorally  by  the  VuU.  which 
may  be  closed  down  over  it,  and  the  eye-lashes. 

But  to  provide  for  its  rotation  in  its  orbit,  each  eye  has  six  ex- 
trinsic muscles.  These  are  arranged  in  three  antagonistic  pairs,  so 
that  for  any  direction  of  rotation  there  is  a  motor  muscle  and  a  check 
muscle,  giving  complete  control  of  its  movements  or  for  holding  it  in 
any  required  position.  These  antagonistic  pairs  of  muscles  are  so 
arranged  that  each  pair  rotates  the  eye  on  one  primary  axis  of  rota- 
tion, such  axes  being  at  right  angles  to  each  other,  and  the  two  mus- 
cles of  a  pair  rotating  the  eye  in  opposite  directions. 

Horizontally,  the  eye  is  rotated  rightward  or  leftward  on  a  ver- 
tical axis  of  rotation  by  two  opposing  muscles,  the  internal  and  ex- 
ternal recti  muscles.  Vertically,  it  is  rotated  upward  or  downward 
on  a  horizontal  axis  by  two  opposing  muscles,  the  superior  and  infe- 
rior recti  muscles.  Torsionally,  it  is  rotated  or  twisted  with  or 
against  the  direction  of  the  clock  hands  by  two  opposing  muscles,  the 
superior  and  inferior  oblique  muscles.  Each  eye  is  supplied  with  a 
full  set  of  these  muscles,  arranged  as  described.  But  by  a  combina- 
tion of  the  different  pairs,  it  may  be  rotated  in  any  direction  or  on 
any  axis  of  rotation. 


84  MUSCLES   OF  THE   EYE 

The  muscles  are  all  of  the  longitudinal  variety  and  attached  or 
inserted  at  one  extremity  to  a  cartilaginous  process  of  an  orbital  bone, 
and  this  is  its  stable  anchorage.  At  the  other  extremity  it  is  at- 
tached to  the  eye  by  thread-like  tendons.  The  four  recti  muscles 
and  the  superior  oblique  have  their  anchorage  at  the  posterior  aper- 
ture, the  optic  foramen,  or  to  a  cartilaginous  ring  that  surrounds  it. 
The  four  recti  muscles  extend  forward,  over  or  around  the  eye-ball, 
and  are  attached  to  the  eye-ball  just  back  of  the  corneal  border.  The 
superior  oblique  passes  through  a  cartilaginous  loop,  or  trochlea,  at 
which  it  is  deflected  over  the  eye  near  its  equator  and  is  attached  to 
the  temporal  side  of  the  eye-ball.  The  inferior  oblique  is  anchored 
to  a  nasal  bone  of  the  orbit  and  passes  under  and  around  the  eye  to 
its  temporal  side  where  it  is  attached  to  it.  The  superior,  inferior 
and  external  recti  and  the  inferior  oblique  muscles  are  supplied  by 
branches  of  the  motor  oculi  or  3d  cranial  nerves,  the  superior  oblique 
by  the  4th  and  the  external  recti  by  the  6th  nerves.  But  there  are 
motor  nerves  from  the  cervical  vertebrae  that  influence  these  mus- 
cles. It  may  be  that  there  are  even  other  unsuspected  sources  of 
innervation  and  all  are  subject  to  the  "inductive"  influences  of  the 
intrinsic  muscles,  as  well  as  the  latter  to  the  former. 

Binocular  Pairs 

The  most  important  pairing  of  the  ocular  muscles  is  their  binoc- 
ular arrangement  into  pairs,  and  such  pairing  depends  upon  whether 
the  binocular  rotations  to  be  effected  are  in  the  same  or  in  opposite 
directions.  To  make  these  classifications  clear,  rotations  of  the  eyes 
in  the  same  direction,  and  for  equal  amounts,  are  termed  Versions; 
but  rotations  in  opposite  directions,  also  for  equal  amounts,  are 
termed  Ductions.  But  since  muscular  force  is  often  applied  to  the 
eyes  for  the  purpose  of  preventing  rotations,  the  terms  are  used  to  in- 
dicate the  muscular  tensions,  rather  than  to  the  rotations.  The  object 
of  the  versions  is  always  to  widen  the  field  of  vision;  that  of  the  duc- 
tions is  always  to  fuse  the  images,  or  put  them  in  the  correct  rela- 
tive positions  for  fusion.    They  would  be  defined  as  follows : 

I.  VERSION: 

That  voluntary  muscular  action  that  normally  rotates  the 
two  eyes  equally  in  the  same  direction. 


MUSCLES   OF  THE   EYE  85 

2.  DUCTION: 

That  involuntary  muscular  action  that  normally  rotates  the 
two  eyes  equally  in  opposite  directions. 

With  respect  to  the  rotations  of  the  eyes,  since  either  a  version 
or  a  duction,  to  be  definitely  recorded,  must  have  a  starting  point  or 
datum,  it  is  important  to  note  that,  for  the  versions,  the  datum  or 
data  are  naturally  the  Medial  Planes.  But,  as  the  ductions  are 
strictly  according  to  the  relative  directions  of  the  visual  axes,  and 
their  parallelism  is  the  primary  relationship  for  viewing  distant  ob- 
jects, Parallelism  of  tJie  Visual  Axes  is  the  datum  for  the  ductions. 
This  does  not  imply  that,  when  the  visual  axes  are  parallel,  there  is 
no  duction  force  being  exercised,  for  a  duction  or  a  version  is  the 
muscular  tension  exercised  for  a  given  rotational  movement,  and  not 
the  movement  itself. 

The  Medial  Planes 

To  be  made  the  datum  for  rotations  of  the  eyes,  versions  or 
ductions,  the  medial  planes  must  be  of  a  stable  position,  at  least  as 
far  as  they  can  be  made  so.  We  therefore  regard  them  as  pertain- 
ing to  the  bony  orbits  of  the  eyes,  rather  than  to  the  mobile  eyes. 
It  is  assumed  further  that  they  pertain  to  these  orbits  when  the  head 
is  erect,  and  at  normal  poise  on  the  neck  and  shoulders,  so  that  the 
eyes  are  in  the  normal  position  for  straight  away  vision.  These 
planes  are  then  located  as  follows  : 

1.  The  VERTICAL  Medial  Plane  is  vertical  and  bisects  the 

horizontal  straight  line  connecting  the  centers  of  rotation  of 
the  eyes. 

2.  The  HORIZONTAL  Medial  Plane  is  horizontal  and  passes 

through  both  centers  of  rotation  of  the  eyes,  such  centers 
being  supposed  to  coincide  with  the  orbital  centers. 

3.  The  INTERORBITAL  Medial  Plane  is  at  right  angles  to 

each  of  the  other  two.  It  may  pass  through  both  centers 
of  rotation  of  the  eyes,  or  be  moved  forward  to  any 
position. 

The  2d  as  well  as  the  3d  is  interorbital,  and  the  3d  as  well  as  the 
first  is  vertical,  but  these  are  subordinate  features  of  them.  As  they 
all  pertain  to  the  orbits,  rotations  of  the  eyes  in  the  orbits  do  not 


86 


MUSCLES    OF   THE    EYE 


affect  their  positions.  Inclinations  of  the  head  or  body  would  do  so, 
but  we  regard  them  as  fixed  in  the  positions  described  whatever  the 
movements  of  the  head  or  body.  The  eyes  may  be  considered  as 
having  medial  planes,  but  the  medial  planes  of  the  eyes  are  of  no 
value  as  data  to  indicate  movements  or  tensions.  The  eyes  may  be 
rotated  to  the  right  or  left,  up  or  down,  or  torsionally,  with  refer- 
ence to  these  planes,  and  such  movements  are  "versions"  if  equal 
and  in  the  same  direction;  but  they  are  "ductions"  when  equal  and 
in  opposite  directions. 

Primary  Position 

The  primary  binocular  position  of  the  eyes  is  that  position  in 
which  the  visual  axes  are  parallel  with  each  other  and  both  are  par- 
allel with  the  1st,  coincident  with  the  2d,  and  perpendicular  to  the  3d 
medial  plane  as  above  described.  But  this  is  a  position  merely  with- 
out regard  to  what  muscular  tensions  may  be  necessary  to  fix  the 
eyes  in  that  position,  or  how  great  or  little  muscular  force  will  be 
necessary  to  rotate  them  out  of  it.  If  they  are  forced  into  that  posi- 
tion by  muscular  action,  the  term  "orthotropia"  is  sometimes  applied 
to  it,  but  as  it  is  a  self-contradictory  term,  it  is  not  much  used. 

But,  if  the  above  described  primary  position  is  maintained  easily 
and  comfortably,  without  special  tension  on  one  pair  of  the  extrinsic 
muscles  more  than  any  other  pair,  or  is  the  position  of  muscular 
rest,  the  eyes  having  no  tendency  to  turn  out  of  it,  either  versionally 
or  ductionally,  we  use  the  term  "Orthophoria"  to  describe  it,  and  that 


FIGURE  22. 

Diagram  of  attachments  of  muscles  to  eyeballs,  arranged  in  version  pairs 
by  number:  1-1,  Hyper-version  pair.  2-2,  Hypo-version  pair.  3-3,  Dextra- 
version  pair.  4-4,  Sinestra-verslon  pair.  5-5,  Right-tor-version  pair.  6-6,  Lieft- 
tor-version  pair.   R,  Right  eye.    L.,  Left  eye.    T-T,  Trochleas.    B-B,  Nasal  bones. 


MUSCLES   OF  THE   EYE  87 

is  a  significant  and  appropriate  term  to  apply  to  it.  It  is  normal  mus- 
cular balance  or  poise  of  the  eyes.  The  primary  position  is  the  posi- 
tion the  eyes  naturally  tend  to  assume  when  all  of  the  muscles  are 
equally  relaxed.  It  corresponds  to  the  term  "emmetropia"  as  applied 
to  the  refraction  of  an  eye  relative  to  its  axial  depth,  something  that 
cannot  be  improved  upon,  but  it  is  a  binocular  term,  whereas  emme- 
tropia  is  a  monocular  term. 

It  is  not  to  be  supposed  that,  because  of  orthophoria,  all  muscles 
are  of  a  standard  length,  or  that  their  attachment  to  the  eye-ball  is 
at  some  standard  fixed  point,  or  even  that  opposing  muscles  are  per- 
fectly equal  in  all  respects.  But  if  any  muscle  is  relatively  long,  or 
is  attached  relatively  too  far  f orv^^ard  or  back  upon  the  eye-ball,  there 
is  an  equal  but  opposite  malattachment  of  its  opposing  muscle  in  the 
other  eye,  so  that  the  net  result,  or  the  combined  eflfect,  is  such  as  to 
balance  the  two  eyes.  There  is  no  such  anomaly  as  a  monocular  im- 
balance of  the  muscles.  The  term,  and  other  terms  of  the  same 
character,  refer  to  the  relationship  of  the  two  eyes,  not  to  their  in- 
dividualities. 

Functional  Activities 

Given  then  a  pair  of  orthophoric  eyes,  the  only  thing  to  consider 
about  their  muscular  duties  is  in  the  exercise  of  their  normal  func- 
tions, and  these  are  their  versions  and  their  ductions,  both  of  which 
are  exercised  by  the  same  muscles,  but  in  diflferent  arrangements  of 
pairs ;  and  both  of  which  are  stimulated  by  the  same  motor  nerves, 
but  upon  an  entirely  different  principle,  the  versions  being  voluntary, 
the  ductions  involuntary.  The  tracks  of  a  railroad  point  out  the  di- 
rection the  train  is  to  proceed ;  but  the  switchman  controls  the  track 
it  must  proceed  upon,  whether  parallel  to  the  main  track  or  in  an 
angular  direction  from  it. 

Normal  Versions 

With  respect  to  terms,  or  the  nomenclature  of  ocular  movements 
or  muscular  tensions,  there  is  a  somewhat  confusing  array  of  terms 
that  would  be  applicable.  But,  notwithstanding  that  fact,  there 
seems  to  have  been  selected,  in  many  cases,  the  wrong  term  for  a 
given  action.  Names  are  not  of  much  account  except  as  representing 
ideas,  and  even  an  inappropriate  name  is  better  than  none,  so  we 


88  MUSCLES   OF  THE   EYE 

won't  quarrel  over  so  simple  a  question  as  "What  shall  we  name  the 
baby?" 

1.  HYPER-VERSION: 

This  is  that  voluntary  muscular  action  that  normally  ro- 
tates both  eyes  equally  upward ;  or  may  restrain  them 
from  turning  in  that  manner  downward. 

2.  HYPO-VERSION: 

This  is  that  voluntary  muscular  action  that  normally  ro- 
tates both  eyes  equally  downward ;  or  may  restrain  them 
from  turning  abnormally  upward. 

In  either  case,  we  may  take  the  action  or  not,  as  we  choose.  If 
we  have  no  interest  in  the  air-plane  that  is  purring  above  our  heads, 
we  are  not  compelled  to  turn  the  eyes  upward  to  see  it.  We  can  let 
it  go  to  some  other  day  if  we  want  to.  Perhaps  we  are  afraid  the 
sun  may  shine  in  our  eyes ;  or  perhaps  there  is  some  other  sight 
nearer  at  hand  but  to  our  right  or  left  we  prefer  to  gaze  at.  As  free 
citizens  we  have  our  choice,  and  there  is  no  subordinate  air-plane 
agent  who  can  compel  us  to  look  up.  This  exercise  of  choice  or  will 
makes  the  act  a  voluntary  one,  or  a  voluntary  non-action.  All  ver- 
sions are  of  that  character.  You  are  welcome  to  use  a  different 
term  to  describe  it,  if  you  choose.  Free  speech  is  voluntary  also, 
and  all  speech  is  voluntary,  if  you  accept  the  penalty  for  saying  in- 
discreet things. 

3.  DEXTRA-VERSION: 

This  is  that  voluntary  muscular  action  that  normally  rotates 
both  eyes  equally  in  a  rightward  direction;  or  restrains 
the  eyes  from  turning  abnormally  to  the  left. 

4.  SINESTRA-VERSION: 

This  is  that  voluntary  muscular  action  that  normally  rotates 
both  eyes  equally  to  the  leftward ;  or  that  restrains  them 
from  turning  abnormally  rightward. 

If  you  are  seated  in  an  amphitheatre  with  an  air-plane  overhead, 
but  a  pretty  girl  at  either  side  of  you,  you  may  look  up  to  the  air- 
plane as  much  as  you  choose,  but  there  is  nothing  to  compel  you  to. 
You  may  find  it  quite  as  entertaining  to  exercise  a  "dextra-version" 


MUSCLES   OF   THE    EYE  89 

or  a  "sinestra-version"  instead.  But  the  main  thing  is  that  you  have 
your  choice,  only  that  you  are  responsible  for  it.  It  might  be  the  part 
of  discretion  to  look  intently  at  the  air-plane. 

These  versions  have  all  been  observ^ed  and  charted,  but  the  ver- 
sions of  the  eyes  around  their  optic  axes,  or  on  their  anterio-posterior 
axes,  have  not  been  charted.  The  eyes  are  so  much  alike  all  the  way 
around  that  it  would  be  difficult  to  tell  whether  they  stood  normally 
erect  or  not.  Their  versions  on  these  axes,  if  ever  charted,  will  be 
as  follows : 

5.  RIGHT-TOR-VERSION : 

That  voluntary  muscular  action  that  normally  rotates  the 
two  eyes  equally  in  the  direction  that  the  hands  of  a  clock 
move,  or  restrains  them  from  the  opposite  turning. 

6.  LEFT-TOR-VERSION: 

That  voluntary  muscular  action  that  normally  rotates  both 
eyes  in  the  direction  opposite  to  the  hands  of  the  clock,  or 
restrains  them  from  turning  the  other  way. 

Version  Co-ordinates 

It  seems  like  a  waste  of  time  and  space  to  point  out  what  muscles 
are  engaged  in  pairs  to  effect  the  above  described  versions,  so  we  will 
be  brief  about  it. 

1.  Hyper-version:  Two  superior  recti  active;  two  inferior  recti 

serving  to  check  the  upward  turning. 

2.  Hypo-version :  Two  inferior  recti  active ;  two  superior  recti 

serving  to  check  the  downward  version. 

3.  Dextra-version :  External  of  right  eye,  internal  of  left  eye; 

with  opposite  pair  checking  movements. 

4.  Sinestra-version :  External  of  left  eye,  internal  of  right  eye ; 

with  opposite  muscles  serving  as  checks. 

5.  Right-tor-version :  Inferior  oblique  of    right    eye,    superior 

oblique  of  left  eye;  opposite  pair  checks. 

6.  Left-tor-version:  Superior  oblique  of  right,  inferior  oblique 

of  left  eye;  opposite  pair  checks. 
The  above  pairs  of  muscles  are  termed  co-ordinates,  as  their 
rotations  of  the  eyes  are  in  the  same  direction  and  for  the  same  pur- 
poses, to  bring  into  the  field  of  observation  as  wide  a  range  as  possi- 


90 


MUSCLES   OF   THE   EYE 


ble.     The  head  and  body  movements  are  also  co-ordinate  to  the 
versions. 


.^<:!^**. 


FIGURE  23. 

Diagram   of  versions  of  the  eyes:    H,   Right  or  dextra-version.    K,  Up  or 
hyper-version.    J,  Combined  leftward  and  downward  version. 

Version  Characteristics 

The  versions  of  the  eyes  are  the  most  natural  of  their  move- 
ments or  muscular  tensions.  If  w^e  had  but  one  eye,  we  would  still 
require  to  make  the  same  movements  to  extend  our  field  of  visual 
observation.  Binocularly  we  make  these  movements  as  naturally  as 
we  would  with  but  one  visual  organ.  But  the  eyes  are  not  dupli- 
cates, for  one  of  them  is  the  right  eye  and  the  other  is  the  left,  and 
they  differ  the  same  as  the  right  and  left  hands  or  feet.  We  refer 
however,  to  their  differences  in  character  from  the  ductions. 

The  eyes  are  a  team,  and  like  the  movements  of  a  team  of  horses, 
their  versions  correspond.  The  motor-nerve  stimulus  that  comes  to 
the  muscles  for  the  versions,  or  the  neuro-motor  center,  is  deep 
seated.  In  controlling  the  direction  of  a  team  of  horses  with  the 
reins,  a  pull  upon  one  rein  turns  both  horses'  heads  together  in  the 


MUSCLES   OF   THE    EYE  91 

same  direction,  and  by  one  impulse.  Our  versions  of  the  eyes  are 
like  that.  To  turn  one  eye  is  to  turn  both  of  them,  and  equally  in 
the  same  direction.  In  the  reins  this  is  brought  about  by  dividing 
each  rein  into  two  segments,  one  going  to  the  right  side  of  the  bit  in 
each  horse's  mouth  and  one  to  the  left  side  of  each.  It  is  the  segments 
of  each  of  the  reins  that  correspond  to  the  eye  muscles,  but  the 
control  is  in  the  one  line  or  one  impulse  along  it. 

But  version  control  of  the  eyes  is  much  more  complete.  We  turn 
the  eyes  in  any  direction,  not  merely  in  two  opposite  directions,  and 
we  are  constantly  exercising  this  control.  The  versions  are  the  prim- 
ordial rotations  of  the  eyes.  We  cannot  negotiate  the  version  of  one 
eye  alone  without  the  other  "going  along  with  it".  Otherwise  it  is  not 
a  version.  Cover  one  eye  and  change  the  direction  of  the  object  for 
the  uncovered  eye ;  and  back  of  its  cover  the  covered  eye,  which  does 
not  see  the  object,  will  rotate  with  the  uncovered  eye  that  does  see  it. 
There  is  unison  of  rotation  of  the  two  eyes.  It  is  not  impossible  to 
break  this  unison  of  motion,  for  otherwise  we  could  have  no  ductions, 
but  the  natural  unison  of  the  two  resists  or  opposes  their  separation 
of  movement. 

The  range  of  the  versions  is  wide.  Normally,  for  an  object  in 
a  direction  of  45°  from  the  vertical  medial  plane  to  the  right,  a  ver- 
sion of  that  amount  may  be  exercised.  Such  a  version  is  the  equiva- 
lent of  lOoA,  for  it  amounts  to  100  cm.  in  a  meter's  distance.  But 
leftward  the  eyes  may  be  deviated  an  equal  amount  or  100 A.  This 
makes  a  total  horizontal  version  of  200 A,  while  maintaining  binoc- 
ular single  vision.  We  prefer  the  terms  "dextra"  and  "sinestra"  ver- 
sion for  these  movements  to  the  terms  "ab"  and  "ad"  version ;  for  to 
the  rightward  it  would  be  "abversion"  of  the  right  eye  and  "adver- 
sion"  of  the  left  eye.  But  there  appears  to  be  neither  ab  nor  ad  to  a 
version,  with  the  two  eyes  turning  together  in  the  same  direction. 
Those  prefixes  are  appropriate  for  the  ductions  only.  Vertically 
there  is  about  the  same  version  power,  but  the  amounts  are  unimpor- 
tant, since  they  are  entirely  adequate.  The  tor-versions  are  more 
limited,  as  the  oblique  muscles  function  more  importantly  in  steady- 
ing the  eyes  for  all  of  the  other  versions  and  ductions  than  in  making 
specific  turnings  of  them  axially. 

Since  the  versions  are  primordial  and  natural,  it  follows  that 


92  MUSCLES   OF  THE   EYE 

even  a  blind  man,  with  eyes,  will  rotate  them  in  this  manner ;  and 
the  new  born  babe  does  not  have  to  wait  for  hard  experience  to  teach 
him  the  art,  as  he  exercises  it  instinctively,  and  from  birth,  and  quite 
as  competently  as  a  grown-up,  provided  the  eyes  are  normally 
mounted  and  the  muscles  are  normally  attached  so  as  to  produce  the 
condition  known  as  orthophoria,  or  something  not  too  far  departed 
from  it.    Optically,  normal  version  powers  are  expected  of  the  eyes. 

Version  Test 

The  purpose  in  making  a  version  test  of  the  eyes,  is  to  ascer- 
tain the  range  of  their  unison  of  rotation,  indicating  that  both  of  the 
muscles  of  a  version  pair  are  functioning  together.  The  test  may  be 
made  in  a  simple  way  by  holding  a  small  target  a  few  feet  before 
the  eyes,  having  the  patient  fix  it,  and,  instructing  him  to  hold  the 
head  stationary,  move  the  target  horizontally  and  vertically  while  ob- 
serving the  rotations  of  the  eyes  to  follow  it.  The  fact  that  he  is 
converging  to  a  near  object  will  have  but  slight  effect  upon  the  ampli- 
tude of  movement,  and  at  the  extreme  points.  If,  in  such  a  test,  one 
of  the  eyes  slows  up  or  stops,  the  muscle  that  has  ceased  to  function 
is  located.  It  is  the  muscle  that  fails  to  rotate  that  eye  in  the  direc- 
tion of  the  target. 

This  test  may  be  reduced  to  a  visual  one  by  using  a  small  light 
as  the  target  and  covering  one  eye  with  a  red  disc.  If  he  fixes  the 
target  the  images  will  fuse  as  a  combination  of  red  and  white,  or 
appear  as  a  single  pink  light.  Then  in  moving  the  target  as  de- 
scribed, it  should  continue  to  appear  as  a  single  pink  light.  Sepa- 
ration of  the  two  lights  at  any  point  indicates  that  one  of  the  eyes  is 
lagging  behind  the  other  or  that  it  may  have  stopped  altogether.  The 
eye  that  lags  or  stops  will  see  its  target  (red  or  white)  move  faster 
in  the  given  direction,  taking  a  position  in  advance  of  it.  Therefore, 
at  the  point  where  diplopia  appears,  version  of  the  eyes  has  ceased, 
for  they  are  no  longer  rotating  equally  in  the  same  direction.  The 
non-functioning  muscle  lies  on  that  side  of  the  eye  that  lags,  toward 
which  movement  is  being  made. 

For  example,  if  the  red  disc  is  before  the  right  eye,  and  the  dis- 
tance of  the  target  is  one  meter,  and  a  rightward  movement  may  be 
made  of  50  cm.  without  diplopia,  this  is  50A  of  a  version  in  that 


MUSCLES   OF  THE   EYE  93 

direction.  But  if  at  60  cm.  to  the  right,  the  red  light  appears  farther 
to  the  right  than  the  white  one,  it  is  the  right  eye  that  is  lagging  be- 
hind the  left,  or  has  stopped  altogether,  and  it  is  its  external  rectus 
that  has  ceased  to  function  at  the  50  cm.  distance.  If  a  leftward 
movement  of  the  target  produces  a  break  to  diplopia  at  40  cm.  and 
the  red  light  again  goes  ahead  of  the  white  one,  it  is  again  the  right 
eye  that  lags,  and  this  shows  that  its  internal  rectus  will  not  function 
beyond  the  point  indicated,  40A  to  the  leftward.  The  eye  that  sees 
the  target  move  the  faster  or  the  farther  is  the  stationary  or  lagging 
eye.  ' 

It  may  be  that  there  will  be  initial  diplopia,  or  that  a  red  and 
a  white  light  will  at  once  appear,  and  this  indicates  a  muscular  im- 
balance with  which  we  are  not  at  present  concerned.  It  is  only  nec- 
essary then  to  observe  the  effects  of  the  versions  called  for  by  the 
movements  of  the  target  upon  the  relative  positions  of  the  two  lights. 
If  version  is  normal  in  every  direction,  the  two  lights  seen  (red  and 
white)  will  maintain  their  fixed  relative  places,  showing  that  the 
versions  are  normal  notwithstanding  the  initial  diplopia.  If  the  eyes 
have  binocular  single  vision,  initial  diplopia  because  of  one  of  the 
lights  being  given  a  different  color  than  the  other,  is  not  usual,  even 
if  there  is  any  muscular  imbalance  to  account  for  it. 

In  the  above  version  tests,  which  may  be  duplicated  for  vertical 
movements,  the  right  eye  will  be  able  to  maintain  fixation  of  the  tar- 
get for  a  greater  movement  to  the  right,  and  the  left  eye  for  a 
greater  movement  to  the  left ;  but  it  is  not  necessary  to  carry  the 
test  to  these  extremes.  If  fusion  is  maintained  for  50  cm.  in  a  test- 
ing distance  of  one  meter,  it  may  be  assumed  that  the  horizontal  ver- 
sions are  normal ;  and  a  range  of  50  cm.  up  and  50  cm.  down  may  be 
regarded  as  normal  for  the  vertical.  In  a  testing  space  of  i  meter, 
these  centimeters  reduce  to  prism  diopters,  which  are  represented  by 
the  sign,  A. 

Version  Anomalies 

The  structure  or  attachment  of  the  ocular  muscles  may  be  such 
as  to  give  them  an  abnormal  tendency  to  deviate  from  their  primary 
positions,  either  up  or  down,  to  right  or  left,  or  torsionally.  For 
versional  directions  of  such  tendencies,  these  would  be  called 


94  MUSCLES    OF   THE   EYE 

The  Homophorias 

There  are  of  course  as  many  kinds  as  there  are  primary  direc- 
tions of  deviation,  and  they  are  named  accordingly.  The  term 
"phoria",  wherever  used  in  the  nomenclature  of  the  muscles,  indi- 
cates a  tendency  of  the  eyes  to  abnormally  deviate  in  some  direction ; 
but  out  of  the  miscellaneous  collection  of  prefixes,  hyper  and  hypo, 
super  and  supra  and  sub  or  infra,  ana  and  kata,  etc.,  we  select  what 
we  consider  to  be  the  most  appropriate  ones  only. 

1.  HYPER-PHORIA: 

A  tendency  of  the  eyes  to  deviate  abnormally  but  equally 
upward,  most  usually  designated  "anaphoria". 

2.  HYPO-PHORIA: 

A  tendency  of  the  eyes  to  deviate  abnormally  but  equally 
downward,  sometimes  called  "kataphoria". 

3.  DEXTRA-PHORIA: 

A  tendency  of  the  eyes  to  deviate  abnormally  but  equally 
to  the  right  or  rightward. 

4.  SINESTRA-PHORIA: 

A  tendency  of  the  eyes  to  deviate  abnormally  but  equally 
to  the  left  or  leftward. 

5.  RIGHT-TOR-PHORIA: 

A  tendency  of  the  eyes  to  rotate  axially  in  the  direction  of 
the  clock  hands,  as  seen  by  patient. 

6.  LEFT-TOR~PHORIA: 

A  tendency  of  the  eyes  to  rotate  axially  in  the  direction 
opposite  to  the  clock  hands. 

As  these  are  but  tendencies,  it  is  assumed  that  the  ocular  muscles 
interpose  and  prevent  such  abnormal  rotations.  This  action  engages 
the  version  pair  of  muscles  that  would  normally  rotate  the  eyes  to- 
gether in  the  opposite  direction.  Thus  for  the  ist,  it  is  the  inferior 
recti  muscles  that  restrain  the  eyes  from  turning  abnormally  up- 
ward. This  action  would  produce  a  constant  strain  upon  these  mus- 
cles. Therefore,  to  relieve  the  muscles,  the  head  may  be  tilted  for- 
ward, so  as  to  allow  the  eyes  to  turn  upward  while  looking  horizon- 


MUSCLES   OF   THE   EYE 


95 


tally  ahead.  In  the  same  or  analogous  manner,  the  2d  would  cause 
a  strain  upon  the  superior  recti  and  this  is  relieved  by  tilting  the 
head  backward.  For  the  same  reason,  in  dextra-phoria,  the  head  is 
turned  to  the  leftward,  or  in  sinestra-phoria  to  the  rightward. 

As  these  abnormal  tendencies  or  turnings  do  not  impair  fusion 
nor  disturb  binocular  single  vision  or  fixation,  they  are  not  given 
much  attention.  A  version  test  would  show  that  the  eyes  can  be 
turned  farther  in  the  direction  of  the  "phoria''  than  in  the  opposite 
direction;  for  the  tendency  is  a  help  in  the  first  direction,  but  a 
handicap  in  the  second.  There  are  occasional  cases  in  which  these 
"homophorias"  or  one  of  them  may  be  very  bothersome  and  dis- 
tressing, and  the  "hyper"  or  "ana"  phoria  is  the  one  most  frequently 
found  of  that  character.  The  only  optical  relief  for  it  would  be  a 
pair  of  prisms,  base  down,  over  both  eyes.  It  is  usually  complicated 
with  other  muscular  anomalies,  and  any  operation  is  likely  to  intro- 
duce this  complication. 


H. 


K. 


FIGURE  24. 

Character  sketches.  Appearances  of  people  with  Homophorias  of  different 
varieties:  H,  Hyperphoria,  head  tipped  forward.  K,  Hypo-phoria,  head  tipped 
back.     J,  Dertra-phorla,  head  turned  to  left. 


Version  Exercises 

In  making  the  version  tests  heretofore  described  we  are  exer- 
cising the  muscles  in  version  pairs.  In  our  every  day  experiences, 
whether  stationary  or  moving  about,  the  eyes  are  constantly  roving 
from  point  to  point,  and  one  might  consider  that  this  was  exercise 
enough.    But  for  the  same  reason  that  w'e  have  to  be  encouraged  to 


96  MUSCLES   OF  THE   EYE 

take  due  care  of  our  health  by  muscular  exercise,  such  as  walking, 
riding,  traveling,  there  is  a  possibility  that,  from  want  of  sufficient 
exercise,  or  a  too  limited  range  of  exercise,  the  ocular  muscles  fall 
into  the  same  dormant  state  that  the  other  muscles  of  the  body  are 
sure  to  acquire  from  the  lack  of  sufficient  physical  exercise. 

To  encourage  physical  exercise  we  use  dumbbells,  trapezes, 
swings,  spring  boards  or  a  whole  gymnasium  equipment  and  make 
periodical  visits  to  it  for  that  purpose.  But  such  exercise  is  greatly 
stimulated  by  the  presence  of  others  who  are  taking  the  same 
measures  as  ourselves  to  develop  their  muscular  powers.  Swimming, 
skating,  rowing,  running,  horse-back  riding  and  other  out-door  exer- 
cises are  indulged.  Theodore  Roosevelt  would  shoulder  his  axe, 
march  into  the  forest  and  cut  down  and  trim  a  big  dead  tree,  for  he 
loved  trees  too  well  to  sacrifice  live  ones  for  that  purpose.  But  our 
greatest  means  of  stimulation  is  competitive  games,  such  as  base-ball, 
tennis,  golf,  foot-ball  and  kindred  "sports"  not  to  mention  arduous 
hunting  and  fishing  trips  to  the  northern  fastnesses,  where  we  endure 
hunger  and  cold  to  accomplish  our  ends,  and  the  primary  purpose  is 
nothing  but  "exercise". 

Wherever  we  go  and  whatever  we  do,  the  ocular  muscles  are  in  a 
constant  state  of  activity.  But  ordinarily  the  range  of  action  is  not 
as  wide  as  necessary.  Therefore  it  is  well  to  devise  a  systematic 
course  of  exercise  for  the  development  of  these  muscles.  The  late 
Charles  H.  Taylor,  of  Yankton,  S.  D.,  although  styled  "Doctor  Tay- 
lor", was  not  an  M.  D.,  at  least  in  practice,  but  an  optometrist.  He 
devised  a  system  of  muscle  exercise  for  the  eyes,  to  which  he  gave 
the  euphoneous  name  of 

"Oculo-Didactics" 

It  is  probable  that  this  would  have  been  better  understood  under 
a  simpler  title,  but  that  point  may  be  questioned.  He  was  not  satis- 
fied to  see  a  single  pair  of  eyes  exercising  in  unison,  but  wished  to 
see  dozens  or  hundreds  of  them  working  together,  so  that  he  had 
version  of  each  of  the  pairs,  as  well  as  version  of  the  hundreds  of 
pairs  together.  For  the  purpose  he  introduced  these  exercises  into 
schools,  and  hundreds  of  pupils,  uniformed  or  in  gym  dress,  would 
go  through  with  the  exercises  together.  It  was  an  attractive  sight, 
and  he  won  the  confidence  and  approval  of  school  superintendents. 


MUSCLES   OF   THE    EYE  97 

principals,  teachers  and  "the  kids"  as  well,  wherever  he  gave  them. 
Naturally  this  won  their  patronage  also,  and  there  are  more  peoi)le 
wearing  Dr.  Taylor's  glasses  in  South  Dakota  than  are  wearing 
everybody  else's  put  together. 

The  doctor  also  interested  many  optometrists  in  the  exercises, 
especially  in  Oklahoma,  where  he  traveled  for  a  time,  giving  tliem 
instructions  in  the  art  of  oculo-didactics.  To  make  one  prom- 
inent in  practical  optometry  he  must  ride  a  hobby,  and  there  is  no 
objection  to  it,  if  the  hobby  is  a  good  one.  A  good  deal  of  the  at- 
tractiveness in  an  attractive  personality  is  featured  most  prominently 
in  the  eyes  and  their  activities,  and  exercises  of  this  kind  add  to  their 
attraction.  But  Dr.  Taylor  was  as  "euphoneous"  in  his  description 
of  the  benefits  to  be  obtained  from  the  exercises  as  in  the  title  he  gave 
them.  Seeing  a  lady  enter  a  store,  a  man  walking  along  before  him, 
or  a  school  girl  picking  her  w^ay  across  a  muddy  street,  he  would 
see  in  their  movements,  the  style  of  walk,  signs  of  muscular  "rigid- 
ity" to  be  overcome  by  oculo-didactics. 

Character  Studies 

One  should  not  be  too  hasty  in  forming  his  opinion  of  a  person's 
character  by  the  way  he  carries  his  head,  or  by  some  peculiarity  in 
his  way  of  looking  at  you.  The  'phorias  we  have  just  described  may 
account  for  some  of  them. 

If  your  new  acquaintance  walks  along  the  street  with  his  head 
tilted  up  or  back,  it  may  be  because  he  is  vain  or  proud  or  thinks  too 
much  of  himself,  but  it  is  possible  that  he  has  Hypophoria,  or  a  ten- 
dency of  the  eyes  to  turn  downward,  due  to  some  structural  mal- 
attachment  of  the  inferior  recti  too  far  forward  on  the  eyes,  or  of  the 
superior  recti  too  far  back,  and  that  it  causes  a  perpetual  strain  or 
tension  on  the  superior  recti  to  hold  the  eyes  level  in  their  orbits.  By 
tilting  the  orbits  upward,  which  he  does  by  tilting  the  head  back,  he 
relieves  this  tension  and  strain,  and  may  be  quite  unconceited  and 
humble  notwithstanding  appearances  being  against  him. 

If  you  become  acquainted  with  someone  who  looks  at  you  from 
under  his  eyebrows,  tilting  his  head  forward  for  the  purpose,  you 
may  regard  his  looks  as  furtive,  and  that  he  has  some  villainy  in  con- 


98  MUSCLES    OF   THE    EYE 

templation,  or  that  he  meditates  attacking  you  forthwith.  But  it  may 
be  that  he  is  afflicted  with  a  Hyperphoria,  or  a  tendency  of  the  eyes 
to  turn  abnormally  upward  in  their  orbits.  Therefore,  he  tilts  the 
head  forward  to  allow  his  eyes  to  assume  a  pose  that  is  restful  to  the 
muscles  that  otherwise  would  be  overtaxed  to  hold  the  eye  down  to 
the  level  position.  Your  life  and  your  pocketbook  may  be  perfectly 
safe  from  violence  as  amiability  and  good  fellowship  may  be  his 
real  characteristics.    Give  him  at  least  the  benefit  of  the  doubt. 

You  may  meet  a  man  or  a  woman  who  casts  "goo  goo"  eyes  at 
you,  turning  the  face  to  right  or  left  away  from  you,  although  look- 
ing into  your  eyes  as  straight  as  anybody.  Don't  conclude  that,  if  it 
is  a  man,  he  is  avoiding  your  eagle  eye,  fearing  it  will  penetrate  his 
character ;  or,  if  it  is  a  woman,  she  is  trying  to  get  up  a  flirtation  with 
you  at  the  first  meeting.  People  who  carry  the  head  habitually  in 
this  manner  and  appear  to  be  averting  your  penetrating  scrutiny,  are 
only  trying  to  get  a  little  relief  from  the  tension  on  a  pair  of  muscles 
by  turning  the  orbits  to  such  position  that  the  muscles  are  relieved  of 
the  task  of  holding  the  eyes  in  their  primary  position.  Dextraphoria 
or  Synestra-phoria  may  account  for  the  whole  matter. 

You  will  meet  some  people  who  tilt  their  heads  sideways,  the 
forehead  to  the  right  and  the  chin  to  the  left.  In  that  way  they  re- 
lieve the  obHque  muscles  of  an  abnormal  strain,  or  perhaps  they  have 
oblique  astigmatism  and  can  see  better  when  the  head  is  tilted  one 
way  or  the  other.  It  may  indicate  a  "coy"  disposition,  bashfulness, 
embarrassment,  but  you  had  better  consult  their  other  features,  their 
language  and  especially  their  actions,  rather  than  pin  your  faith  to 
appearances  only.  Dr.  Taylor  might  explain  these  postures  on  the 
theory  of  an  abnormal  rigidity  of  some  of  the  ocular  muscles,  but 
without  Such  rigidity  of  the  head  and  neck  muscles  the  strain  would 
come  on  the  ocular  muscles  which  are  less  able  to  bear  it. 

Character  reading  from  external  signs  and  symptoms  is  im- 
mensely enhanced  by  studies  of  the  eyes ;  but  no  external  signs  are 
really  infallible.  The  eyes  contribute  a  very  important  element  to  the 
study,  but  such  signs  may  be  taken  as  initial  merely,  something  to  be 
later  explained  by  any  of  the  muscular  abnormalities  included  in  the 
Homophorias,  as  well  as  by  other  signs  not  yet  considered.     There 


MUSCLES   OF  THE   EYE  99 

is  little  the  optometrist  can  do  for  these  muscular  anomalies,  except 
along  the  lines  of  exercise,  or  by  Oculo-Didactics,  as  Dr.  Taylor  has 
christened  them. 


100  MUSCLES   OF  THE   EYE 

CHAPTER  VII 

Duction  of  Eyes 

The  term  "duction"  as  applied  to  the  eyes  has  already  been  de- 
fined as  "that  involuntary  muscular  action  that  normally  rotates  the 
two  eyes  equally  in  opposite  directions".  It  is  the  muscular  function 
that  controls  the  relative  positions  or  directions  of  the  visual  axes, 
and  is  essential  to  the  binocular  fixation  of  objects  at  different  dis- 
tances from  the  eyes,  near  and  far,  and  v^ithout  regard  to  their  di- 
rection from  the  eyes. 

In  binocular  fixation,  which  is  essential  to  the  fusion  of  the  images 
and  single  vision,  the  two  visual  axes  must  meet  at  the  objective 
point  of  fixation,  whether  far  or  near.  Light  from  it  will  then  be  fo- 
cused at  the  center  of  the  fovea  centralis  of  each  eye,  which  are  the 
subjective  points  of  fixation.  As  the  eyes  are  separated  by  the  space 
between  their  rotary  centers,  to  fix  a  near  object  they  must  be  rela- 
tively in  a  slightly  different  position,  turned  inward,  in  which  the 
right  eye  is  turned  to  the  left  and  the  left  eye  is  turned  to  the  right. 
In  changing  this  relative  position  for  the  fixation  of  a  more  distant 
point,  the  right  eye  must  be  turned  to  the  right  and  the  left  eye  to 
the  left.  These  are  duction  movements  of  the  eyes.  They  are  the 
only  normal  ductions. 

But  on  account  of  the  obliquity  of  the  direction  of  the  object, 
one  eye  may  be  required  to  rotate  less  than  the  other,  or  even  one 
may  be  required  to  remain  stationary  or  fixed  while  the  other  eye  exe- 
cutes the  entire  rotation.  This  is  a  combined  version  and  duction, 
and  the  two  functions,  when  exercised  together,  must  not  be  con- 
fused with  one  another.  Version  takes  care  of  the  obHquity  of  the 
direction  of  the  object  fixed,  duction  with  its  nearness  to  the  eyes.  In 
the  instance  referred  to  above,  when  one  eye  remains  stationary,  both 
duction  and  version  are  participated  in  by  both  eyes,  but  for  the 
single  stationary  eye  the  two  neutralize  each  other. 

Contest  of  Functions 

In  the  execution  of  a  duction  of  any  variety  inward,  outward, 
upward  or  downward,  the  functions  of  version  and  duction  are  in 
conflict.  The  rotations  of  the  two  eyes  are  naturally  and  primordi- 
ally  in  the  same  direction,  and  the  movement  is  voluntary.     But,  in 


MUSCLES   OF   THE   EYE  101 

a  duction  movement,  this  rule  of  co-ordination  must  be  violated.  It 
is  a  conjugate,  rather  than  a  co-ordinate  movement.  As  we  accommo- 
date to  conjugate  an  objective  point  focally  with  the  retina,  we  exe- 
cute a  duction  to  conjugate  the  two  retinal  images  of  a  single  object 
or  place  them  in  the  relative  positions  for  fusion.  Hence  the  mus- 
cles that  execute  a  duction  must  overcome  the  version  tendencies  of 
the  eyes  to  rotate  in  the  same  direction  as  well  as  to  rotate  them  in 
opposite  directions. 

It  is  on  this  account  that  duction  movements  of  the  eyes  are  so 
much  more  limited  in  scope  than  the  versions.  While  we  may  easily 
execute  a  version  to  the  right  or  left  of  lOoA  or  more,  we  cannot 
usually  rotate  the  two  eyes  inward  more  than  25 A,  unless  by  exer- 
cise we  develop  greater  duction  power  in  that  direction.  But  in  an 
outward  direction,  the  effectiveness  of  a  duction  in  rotating  the  eyes 
is  very  much  more  limited,  since  about  8A  is  the  limit  of  such  duc- 
tion movements.  In  a  vertical  direction,  the  duction  movements  are 
still  more  limited,  for  turning  the  right  eye  higher  than  the  left  or 
the  left  higher  than  the  right  is  often  limited  to  2 A,  and  is  seldom 
above  5 A.  The  capacity  of  the  muscles  to  execute  rotations  is  very 
much  greater  than  this,  as  shown  by  their  versions,  but  for  executing 
the  ductions  the  version  pairs  of  muscles  are  perpetually  resisting 
duction. 

It  must  not,  therefore,  be  thought  that,  because  a  particular 
duction  is  weak  or  ineffective,  the  muscles  that  execute  it  are  weak. 
The  duction  pair  of  muscles  has  not  been  taught,  as  they  must  be  by 
experience,  to  rotate  the  eyes  in  that  unusual  manner ;  nor  have  the 
version  pairs  that  offer  a  natural  resistance  to  such  movements  been 
taught  to  be  submissive  to  them.  A  farmer  who  drives  to  the  city 
often  will  get  his  team  quickly  out  of  the  path  of  a  fire  engine ;  but 
one  who  seldom  goes  to  the  city  and  is  unused  to  its  ways,  will  not 
be  so  quick  to  take  the  alarm  or  move  his  rig  out  of  the  path  of  the 
fire-fighters.  The  muscles  fall  into  these  special  habits  only  when 
taught  by  experience  that  worse  consequences  follow  their  negli- 
gence or  omission. 

Normal  Ductions 

When  the  eyes  are  normal  and  in  normal  muscular  balance, 
viewing  a  distant  object,  such  as  a  sail  upon  the  horizon,  or  search- 


102  MUSCLES   OF   THE   EYE 

ing  it  for  one,  calls  for  no  duction  of  the  eyes,  and  the  visual  axes 
are  parallel  while  the  versions  are  taking  place.  If  a  sail  is  discov- 
ered, it  is  binocularly  fixed  by  the  paralleling  of  the  visual  axes.  But 
as  it  approaches  the  observer  or  he  approaches  it,  a  slight  degree  of 
convergence  begins  to  be  exercised,  and  convergence  increases  grad- 
ually with  its  approach.  The  visual  angle  is  also  increasing,  and  ac- 
commodation begins  to  be  exercised.  But  all  of  these,  for  a  distance 
such  as  this,  are  very  slight. 

If,  when  within  a  quarter  of  a  mile  of  the  observer,  the  boat  is 
turned  and  sails  away,  the  eyes  are  diverged,  the  accommodation  is  re- 
laxed, and  the  visual  angle  of  the  boat  diminishes  with  its  distance. 
The  convergence  of  the  eyes  for  its  approach  or  nearness  is  an  exer- 
cise of  adduction.  The  divergence  of  the  eyes  with  the  recession  of 
the  object  is  an  exercise  of  abduction.  To  fix  the  object  at  any  dis- 
tance the  two  functions  must  be  balanced  for  that  distance.  They 
oppose  each  other,  and  are  both  opposed  by  the  versions.  For  an 
object  at  20  ft.  or  6  meters,  it  is  not  considered  that  any  convergence 
is  necessary,  although  as  it  is  about  6  centimeters  for  a  distance  of  6 
meters,  it  is  i  cm.  to  the  meter,  or  i  A. 

The  muscles  most  primarily  involved  in  exercising  adduction 
are  the  internal  recti  muscles,  with  the  externals  acting  to  check  ex- 
cessive convergence.  For  abduction  the  external  recti  are  primarily 
active,  the  internals  acting  merely  to  check  over  divergence.  The 
guiding  sensation  that  determines  the  relative  action  of  these  oppos- 
ing muscles  is  the  fusion  of  the  images,  or  single  vision  of  the  object. 
But  for  objects  at  such  a  distance  as  the  above,  both  adduction  and 
abduction  are  of  inappreciable  amounts  or  values.  They  are  sup- 
posed to  begin  only  when  the  object  fixed  is  nearer  than  20  ft.,  unless 
there  is  an  imbalance  of  the  muscles  that  makes  duction  necessary 
for  infinity. 

The  Meter  Angle 

When  the  object  is  at  a  distance  of  but  i  meter  from  the  eyes, 
normal  adduction  for  it  is  the  6  cm.  for  the  distance  of  i  meter,  or 
6 A.  But  to  provide  a  designation  that  corresponds  to  the  i  D.  of 
accommodation  that  is  required  by  each  eye  for  an  object  at  that  dis- 
tance, this  degree  of  convergence  or  adduction  is  called  i  meter- 
angle.    The  meter-angle  is  the  angle  formed  by  the  meeting  of  the 


MUSCLES   OF  THE   EYE 


103 


visual  axes  at  a  point  of  fixation  that  is  at  a  distance  of  i  meter  from 
the  eyes,  or  at  an  objett  that  is  at  that  distance.  If  the  objective 
point  of  fixation  is  in  an  oblique  direction  from  the  eyes,  that  is  not 
regarded  as  ^{hanging  the  value  of  the  meter  angle,  nor  in  lessening 


FIGURE  26. 
Binocular  fixation  of  points  O,  O'  or  O".  R,  right  eye,  L.,  left  eye;  M, 
midway  point  between  centers  of  rotation;  CP,  correction  plane;  M',  midway 
point  in  CP;  D,  D'  and  D",  binocular  direction  of  O,  O'  and  O".  Lines  con- 
verging from  R  and  L.  to  O,  O'  or  O"  are  visual  axes  from  centers  of  rotation 
to  those  objective  points  of  fixation.  The  r  and  1  represent  primary  direction 
of  visual  axes  from  M  or  M'. 


104  MUSCLES   OF   THE,    EYE 

ihe  e(]ual  participation  of  the  two  eyes  in  it.  As  stated  heretofore, 
the  versions  take  care  of  the  obHquity  of  direction  of  the  object  point 
fixed,  the  ductions  of  its  distance  from  the  eyes. 

On  account  of  the  variations  of  different  people  in  the  matter  of 
width  between  the  eyes,  the  meter-angle  is  of  indefinite  angular  value, 
measured  in  degrees,  being  greatest  for  those  whose  eyes  are  most 
widely  separated.  But  since,  whatever  that  distance,  accommodation 
of  I  D.  for  the  distance  of  i  meter  co-ordinates  with  convergence  of 
I  meter-angle  for  the  same  distance,  the  space  between  the  eyes  is  not 
regarded  as  affecting  the  meter-angle  in  an  optical  sense.  The  fixed 
average  distance  of  6  cm.  provides  the  basis  for  reducing  a  meter- 
angle  to  its  equivalent  in  prism-diopters,  and  vice-versa ;  and  whether 
the  reduction,  one  way  or  the  other,  is  a  little  above  or  below  this 
average  is  regarded  as  a  negligible  question.  In  reducing  meter- 
angles  to  prism-diopters  we  may  allow  for  an  unusually  wide  or  nar- 
row space  between  the  eyes. 

In  the  I  meter-angle  of  convergence  for  an  object  point  at  that 
distance,  a  line  connecting  the  rotary  centers  of  the  eyes  makes  the 
3d  side  of  a  triangle,  in  which  the  visual  axes  are  the  lateral  sides. 
If  a  line  is  drawn  from  the  point  of  fixation  to  the  center  point  of  the 
base  of  the  triangle,  it  will  bisect  the  meter-angle  or  divide  it  into 
two  equal  segments.  The  line  itself  is  the  binocular  direction  of  the 
point  of  fixation  or  object  point.  Whether  it  is  perpendicular  to  the 
base  line,  or  oblique  to  it,  the  angles  at  each  side  of  it  are  equal,  and 
each  represents  the  angular  excursion  that  either  eye  must  make 
from  parallelism  to  the  fixation  point.  Their  excursions  from  the 
primary  positions  are  of  course  unequal,  except  for  a  point  in  the 
medial  vertical  plane,  but  inequality  of  excursion  does  not  alter 
equality  of  adduction  or  abducton,  or  of  convergence  or  divergence. 

If  the  point  of  fixation  is  in  a  plane  i  meter  forward  of  the  eyes 
but  20  cm.  to  the  leftward  of  the  vertical  medial  plane,  a  fixed  nor- 
mal leftward  version  of  the  eyes  is  required  for  its  binocular  direc- 
tion, and  a  fixed  normal  convergence  is  required  for  its  distance,  or 
the  distance  of  the  plane  from  the  eyes.  The  version  is  leftward 
(sinestra-version)  20 A  for  the  object  point  of  20  cm.  in  a  meter's 
distance  from  the  medial  plane,  although  the  left  eye  rotates  but 
17  cm.  leftward  and  right  eye  23  cm.  in  the  same  direction.     The 


MUSCLES   OF  THE   EYE  105 

difference  between  the  two  is  the  amount  of  the  adduction,  which  is 
6A.  If  the  left  eye  is  fixing  a  point  directly  before  it,  and  the  right 
eye  makes  the  entire  excursion  of  6  cm.  (which  is  a  fanciful  suppo- 
sition) still  the  two  eyes  participate  equally  in  the  adduction ;  for  in 
order  to  converge  the  eyes  to  such  point,  the  left  eye  must  remain 
stationary  or  fixed  and  repress  its  natural  tendency  to  turn  leftward 
with  the  right  eye. 

The  Object  Distance 

In  establishing  the  meter-angle  we  place  the  objective  point  of 
fixation  at  a  distance  of  i  meter  from  the  eyes,  but  from  what  point, 
or  datum?  To  be  mathematical  about  it,  it  would  have  to  be  i  meter 
from  the  respective  centers  of  rotation,  or  from  the  base  of  the  tri- 
angle we  have  referred  to,  the  line  connecting  the  rotary  centers,  or 
from  its  center.  But  this  is  an  inaccessible  point  and  therefore  im- 
practicable. An  interpupillary  line,  or  its  center,  would  not  be  much 
better.  Considered  from  a  practical,  rather  than  a  mathematical 
standpoint,  distances  in  front  of  the  eyes  should  be  measured  from 
the  correction  plane  of  the  eyes,  the  position  in  which  lenses  for  the 
correction  of  the  eyes  must  stand  or  be  placed  to  neutralize  distance 
or  measure  optical  defects  of  the  eyes,  as  well  as  to  correct  them. 
This  position  is  at  about  14  mm.  from  the  eyes,  or  forward  of  the 
corneas.  It  is  therefore  14  -|-  12  =  26  mm.  or  2.6  cm.  forward  of 
the  line  connecting  the  rotary  centers,  and  14  -f"  ^^2  =  17K  "^"^• 
forward  of  the  interpupillary  line,  or  its  center. 

Since  this  plane,  rather  than  the  plane  of  the  rotary  centers,  is 
the  actual  datum  for  neutralizing  distances  or  making  optical  correc- 
tions of  the  eyes,  whether  it  suits  us  mathematically  or  not,  and  is 
the  approximate  location  of  the  anterior  principal  foci  of  a  pair  of 
normal  eyes  at  which  lenses  of  any  value  do  not  alter  the  combined 
focal-length  of  the  eye  and  lens,  we  might  as  well  accept  it  and 
measure  distances  forward  of  the  eyes  from  it.  It  will  not  mate- 
rially alter  the  equivalency  of  it  to  i  D.  or  to  lA  to  it  posteriorally, 
and  is  the  exact  equivalent  of  i  meter  anterior  to  it.  It  places  the 
object  at  1  meter  102.6  cm.  forward  of  the  rotary  centers,  instead 
of  100  cm.,  and  this  is  as  close  a  position  as  we  are  able  usually  to 
get.  For  nearer  distances  than  i  meter,  the  same  2.6  cm.  must  be 
added,  which  is  relativelv  greater  for  them  than  for  i  meter.     But 


106  MUSCLES   OF  THE   EYE 

as  our  prisms  for  measuring  the  ductions  are  necessarily  placed  in 
that  position  we  are  making  it  the  actual  point  or  plane  from  which 
to  measure  distances  whether  we  are  pleased  with  it  or  not. 

Convergence  vs.  Adduction 

The  two  above  terms,  as  well  as  divergence  and  abduction,  are 
used  interchangeably  in  the  preceding  pages,  or  as  synonyms.  They 
are  not  really  synonymous,  however,  nor  equivalents  except  in  ortho- 
phoric  eyes.  The  term  "convergence"  is  a  physical  term.  It  ex- 
presses the  relative  positions  or  directions  of  two  lines  that  meet  at 
a  point,  and  is  applied  to  any  two  lines  in  that  relationship.  There 
is  no  suggestion  in  it  of  an  action,  muscular  or  otherwise.  On  the 
other  hand,  the  term  "adduction"  is  a  physiological  term.  It  indi- 
cates in  the  eyes  a  muscular  action  that  normally  converges  the  vis- 
ual axes,  or  that  lessens  or  neutralizes  their  divergence.  If  the 
visual  axes  are  parallel,  they  cannot  be  said  to  be  convergent;  but 
a  considerable  adduction  may  have  been  necessary  to  parallel  them, 
and  must  be  continued  to  preserve  their  parallelism.  Convergence 
is  the  static,  adduction  is  the  dynamic  term.  Divergence  and  abduc- 
tion have  the  same  relationship. 

Since  we  have  defined  the  term  "duction"  as  a  muscular  action, 
rotation  of  the  eyes  is  not  material  to  the  exercise  of  it ;  and  both  ad- 
duction and  abduction,  .or  any  duction  of  the  eyes,  are  merely  duc- 
tions qualified  as  to  their  direction  of  action.  In  practical  optometry 
we  have  a  great  deal  to  do  with  ductions  that  do  not  rotate  the  eyes, 
and  very  much  less  to  do  with  the  ductions  that  do  rotate  them.  It 
is  this  fact  that  makes  it  important  to  define  the  term  as  we  have. 
There  are  other  qualifying  words  in  the  definition  that  will  be  con- 
sidered later.  From  the  definition  it  may  be  seen  that  the  rotations 
of  the  eyes  may  even  be  contrary  to  their  ductions.  But  nevertheless 
all  movements  of  the  eyes  horizontally  are  due  either  to  ductions  or 
versions.  A  pair  of  eyes,  when  fixed  upon  a  point,  are  balanced  in 
abduction  and  adduction,  for  otherwise  they  would  not  remain  in 
that  relative  position.    But  they  may  not  be  normally  so  balanced. 

In  the  exercise  of  adduction,  abduction  is  the  function  that  acts 
as  a  check  upon  oveir(?onvergence  of  the  eyes.  In  exercising  abduc- 
tion, adduction  is  the  check.    And  these  "checks"  that  are  put  upon 


MUSCLES    OF   THE    EYE  107 

either  iire  ductions  also,  for  they  are  "that  involuntary  muscular 
action  that  normally  rotates  the  two  eyes  equally  in  opposite  direc- 
tions", the  same  as  the  ductions  that  are  effective  in  rotating  the  eyes 
in  opposite  directions.  We  lose  sight  of  them  because  they  do  not 
initiate  the  action,  or  because  their  action  is  merely  negative  in  effect. 

Normal  Near  Vision 

An  object  is  not  counted  "near"  unless  it  is  within  reach  of  the 
hands,  so  as  to  be  adjusted  to  different  positions.  It  is  that  distance 
at  which  we  normally  may  engage  both  eyes  and  hands,  as  required  in 
the  various  occupations.  Among  these  are  reading  and  the  trades 
for  men,  or  reading  and  sewing  for  women.  As  reading  is  one  of 
the  most  important  near  vision  occupations,  that  is  the  usual  gauge 
for  near  distance.  The  most  commonly  accepted  reading  distance  is 
at  Yz  meter,  33^  cm,  or  practically  at  13  inches  from  the  eyes.  If 
we  add  the  14  mm.  for  the  position  of  the  correction  plane  in  front 
of  the  eyes,  it  makes  this  distance  about  14  inches,  or  35  cm. 

Some  occupations  and  some  eyes  may  require  a  greater  or  less 
distance,  and  the  preferred  reading  distance  is  modified  to  some  ex- 
tent by  the  length  of  the  arms.  Hence,  14''  or  35  cm.  from  the  eyes 
place  the  object  at  about  Yz  meter  from  the  lenses  that  are  used  to 
neutralize  that  distance,  or  normal  accommodation  for  it  is  3  D. 
For  this  distance,  normal  convergence  is  the  reciprocal  of  ^,  or  3 
meter-angles.  Reduced  to  prism-diopters,  it  is  18 A.  This  is  the 
normal  amount  of  adduction  exercised  for  the  distance.  A  plane  at 
right  angles  to  the  vertical  medial  plane  at  that  position  embraces  all 
points  of  fixation  for  normal  reading  or  near  vision.  But  in  reading 
a  book  at  that  distance,  as  we  visually  pass  over  the  successive  lines, 
it  is  by  the  exercise  of  the  version  functions  that  we  direct  attention 
to  different  parts  of  the  page. 

Printed  books  are  typed  in  such  manner  as  to  provide  us,  at  that 
distance,  with  an  angle  of  vision  sufficient  to  make  the  images  of  the 
printed  words  of  sufficient  size  to  be  plainly  seen.  If  the  type  is  of 
larger  size  it  is  seen  easily  at  a  greater  distance,  requiring  less  ac- 
commodation and  less  convergence;  and  if  it  is  of  a  smaller  size,  it 
may  be  brought  nearer,  provided  our  accommodation  is  able  to  take 
care  of  it  at  such  nearer  distance  and  convergence  may  also  be  in- 
creased in  the  same  degree.     In  a  faint  light,  however,  we  usually 


108 


MUSCLES   OF   THE    EYE 


hold  our  reading  at  a  nearer  distance.  This  increases  the  volume  of 
light  from  every  point  of  the  printed  page  to  the  eye,  but  it  also  ex- 
acts increased  accommodation  and  convergence  or  adduction. 

In  the  hundreds  and  hundreds  of  occupations  and  the  thousands 
upon  thousands  of  men  and  women,  boys  and  girls  that  are  engaged 
in  them  the  exercise  of  these  muscular  functions  of  the  eyes  goes  on 
perpetually.  The  versions  and  ductions  co-operate ;  the  ductions 
conjugate  the  same  as  the  accommodation,  and  both  co-ordinate  with 
each  other ;  and  all  of  them  co-ordinate  with  the  manual  require- 
ments of  the  occupations.  These  include  the  stenographer,  the  store 
clerk,  the  mechanic,  the  factory  hand,  the  shoe-maker  or  cobbler, 
the  candy-maker,  the  doctor,  the  dentist,  the  optometrist  or  optician, 
the  ticket-seller,  the  bank-teller  and  money  changers  generally,  the 
teacher  and  the  preacher.  But  more  than  any  other  class,  the  stu- 
dents at  college  and  the  pupils  in  the  schools,  what  a  vastness  there 
is  to  it  all;  and  most  of  this  occupational  work  is  done  when  the 
other  bodily  muscles  are  at  rest  and  inactive.  But  the  work  of  the 
ocular  muscles  does  not  cease  with  the  ringing  of  a  time  bell,  nor 
when  the  tired  worker  hangs  to  a  strap  in  the  street  car  on  his  way 
home,  or  even  when  he  arrives  at  that  destination. 

A  handicap  to  the  exercise  of  these  muscular  functions  of  the 
eyes,  due  to  some  optical  defect  that  lenses  will  eliminate  and  correct, 
is  too  serious  a  matter  not  to  be  given  prompt  and  efficient  attention, 
and  this  is  the  service  to  humanity  that  the  optometrist  renders.  To 
be  able  to  do  so  efficiently  is  not  beneath  the  dignified  ambition  of  the 
very  highest  human  qualities  and  character.  Let  the  optometrist 
therefore,  regard  his  profession  in  that  light,  and  make  himself 
worthy  of  it. 


FIGURE  27. 

Ductlon  pairs  of  muscles:  1  and  1',  hyperpo-  or  sur-sum-duction;  2  and  2', 
hypoper-  or  sum-sur-duction;  3  and  3',  adduction;  4  and  4',  abduction;  5  and 
5',  supracyclo-duction ;  6  and  6',  infracyclo-duction. 


MUSCLES   OF   THE    EYE  109 

Fixation  of  Object 

In  "searching"  the  objective  field  for  an  object  either  near  or 
far,  the  visual  axes  are  adapted,  in  relative  positions  or  directions, 
to  the  direction  of  the  object  as  well  as  to  its  distance.  Thus,  in 
viewing  the  distant  horizon  for  a  sail,  the  visual  axes  are  directed, 
not  to  one  point  of  it,  but  sweep  it  from  point  to  point.  The  object 
is  most  likely  to  be  perceived,  at  first,  by  indirect  vision,  but  imme- 
diately we  catch  sight  of  it  in  that  way,  direct  vision  is  turned  to  it, 
so  that  its  images  may  fall  upon  the  subjective  points  of  fixation  in 
both  eyes  at  once.  The  eyes  will  turn  in  that  direction  first,  and  by 
a  version  movement.  We  may  then  turn  the  head  so  as  to  face  it  and 
see  it  squarely. 

A  sail  upon  the  horizon  that  has  to  be  searched  for  in  that  man- 
ner will  make  but  a  small  image  upon  either  retina,  perhaps  the  5' 
angle  of  vision  whose  tangent  is  .00145.  The  images  will  therefore 
occupy  but  a  small  portion  of  the  fovea  centralis  of  either  eye,  whose 
visual  angle  is  about  68',  and  whose  tangent  value  is  practically  .02. 
By  comparison  either  of  the  angles  or  their  tangents,  it  is  about  1/13 
of  the  diameter  of  the  fovea,  or  1/170  of  its  area.  It  is  not  surpris- 
ing that  fixation  of  so  small  a  target  and  the  maintenance  of  fixation 
upon  it  is  rather  tiring  to  the  muscles  that  are  engaged.  With  the 
approach  of  the  object  it  becomes  easier  for  we  are  not  apt  to  lose 
sight  of  it  merely  because  vision  wanders  to  different  points  of  it. 
The  fixation  distance  changes  but  slowly,  for  convergence  is  but 
slightly  changed  in  the  approach  of  a  boat  from  5  miles  to  one-half 
of  a  mile. 

In  the  same  manner  we  search  for  a  near  point  of  fixation,  such 
as  a  particular  word  or  letter  on  a  page  of  reading  matter,  a  dropped 
coin  on  the  ground,  a  particular  character  in  a  drawing,  a  special 
item  in  a  column  of  "want  ads"  and  so  on  down  to  the  proverbial 
"needle  in  a  haystack".  If  we  know  exactly  what  we  are  looking  for 
we  may  see  it  more  quickly  than  if  it  is  an  unknown  article.  But 
in  such  a  search  and  the  final  discovery  of  what  we  are  searching  for 
and  its  binocular  fixation  the  duction  and  version  functions  are  in 
constant  activity.  When,  as  in  an  hour's  reading,  the  characters  we 
are  fixing  are  constantly  changing  and  must  be  clearly  visualized  as 


no  MUSCLES   OF   THE   EYE 

the  images  "flow"  across  the  retinas  it  is  surprising  that  we  are  able 
to  keep  it  up  hour  after  hour  with  unabated  intensity  and  interest. 

To  wish  to  explore  the  visual  organs  for  the  purpose  of  locating 
the  agencies  for  exercising  the  functions  referred  to  and  finding  more 
details  in  regard  to  them,  is  like  asking  the  captain  of  the  steamer  that 
has  carried  us  to  distant  lands  for  a  permit  to  explore  the  interior  of 
the  ship.  It  has  taken  us,  in  luxury,  across  wide  oceans,  brought  us 
to  distant  cities,  given  us  a  view  of  the  magnitude  of  the  world  and 
displayed  its  wonders,  but  we  still  wish  to  inspect  its  engines,  cargo, 
and  interior  parts,  including  the  culinary  provisions  by  which  we 
have  been  fed,  the  cabins,  sleeping  quarters,  kitchen,  and  the  ship's 
force  of  men  and  women  who  have  kept  things  going,  its  instruments 
for  navigation,  its  wireless  communication  with  the  other  parts  of 
the  world  and  even  the  coal  supply,  the  stokers,  the  engineers  and 
everything  about  it.  Complete  and  competent  as  all  these  are  they 
are  not  so  wonderful  as  the  visual  apparatus  by  which  we  see,  nor 
nearly  so  automatically  operated. 

We  must  know  these  details  in  order  to  know  what  artificial 
means  to  employ  to  overcome  any  weakness  of  functioning,  by  which 
these  high  but  delicate  duties  are  exercised,  and  to  supply  them 
with  the  best  artificial  agents  for  correcting  their  defects,  so  as  to 
give  the  natural  or  normal  functions  the  best  opportunities  possible 
for  the  discharge  of  those  functions  easily  and  comfortably,  for  this 
is  essential  to  the  full  enjoyment  of  the  highest  of  all  human 
senses — vision. 

Duction  Muscles 

In  the  exercise  of  the  ductions  we  have  the  same  orbits  for  the 
eyes  to  rotate  in,  the  same  eyes  to  be  rotated  in  them,  the  same  com- 
plete set  of  extrinsic  muscles  to  operate  them,  and  the  same  system  of 
motor  nerves  to  stimulate  their  action  as  in  the  versions.  But  the 
muscles  are  binocularly  associated  in  different  pairs  than  for  the 
versions,  and  their  nerve  stimulation  is  involuntary,  or  by  reflex 
action  from  ganglionic  nerve-centers.  The  purpose  of  such  action 
(the  ductions)  is  to  obtain  or  to  retain  fusion  of  the  images,  or  to  so 
hold  or  rotate  the  eyes  that  the  retinas  will  stand  in  their  proper 
relative  positions  to  receive  the  images  on  corresponding  areas,  as 
required  for  fusion. 


MUSCLES   OF  THE   EYE  111 

In  classifying  the  ductions  and  the  muscles  that  execute  them, 
the  two  will  be  considered  together;  and  it  will  be  necessary  to  in- 
troduce certain  new  terms,  for  nothing  less  than  a  surgical  operation 
will  suffice  to  correct  some  of  the  faults  of  the  old  ones.  This  ap- 
plies equally  to  the  'phorias  that  make  abnormal  ductions  of  the  eyes 
necessary.  According  to  their  directions  of  tension  or  rotation  of  the 
eyes  upon  their  three  primary  axes  of  rotation,  the  ductions  are  as 
follows: 

I.  Hyperpo-duction,  superior  rectus  of  right  eye  with  inferior 
rectus  of  left  eye. 
■    2.  Hypoper-duction,  inferior  rectus  of  right  eye  with  superior 
rectus  of  left  eye. 

3.  Adduction,  internal  rectus  of  right  eye  with  internal  rectus  of 

left  eye. 

4.  Abduction,  external  rectus  of  right  eye  with  external  rectus 

of  left  eye.  * 

5.  Stipracyclo-duction,  superior  oblique  of  right  eye  with  supe- 

rior oblique  of  left  eye. 

6.  Infracyclo-duction,  inferior  oblique  of  right  eye  with  inferior 

oblique  of  left  eye. 

As  for  the  first  term,  the  term  "sursum-duction"  may  be  sub- 
stituted if  preferred;  and  for  the  second,  the  term  "sumsur-duction" 
as  the  accepted  term  should  clearly  indicate  the  fact  that  two  duction 
muscles  are  the  active  agents,  as  a  duction  cannot  be  performed  by 
one  muscle.  For  example,  when  witli  a  pair  of  orthophoric  eyes,  a 
prism  is  placed,  base  down,  before  the  right  eye,  it  is  obvious  that 
to  maintain  fixation  of  the  object,  the  right  eye  must  be  rotated  up- 
ward. But,  it  is  equally  obvious  that  the  left  eye  must  be  held  in 
its  fixed  position,  or  restrained  from  rotating  upward  with  the  right 
eye,  according  to  its  version  tendencies.  This  puts  the  muscular 
tension  equally  on  the  superior  rectus  of  the  right  eye  and  inferior 
rectus  of  the  left  eye.  If  it  is  not  equal,  the  head  will  be  tilted  to 
make  it  so. 

The  adductions  and  abductions  are  normal  only  to  the  extent 
that  it  is  necessary  to  engage  them  for  the  actual  nearness  of  the 
object,  adduction  for  a  nearer  object,  abduction  for  a  more  distant 


112  MUSCLES   OF  THE   EYE 

one.  Vertically,  the  ductions  are  not  called  into  action  for  the  near- 
ness of  the  object,  nor  for  its  lateral  position.  Such  positions  of  it 
engage  the  muscles  in  version  pairs  only.  The  eyes  are  synchronous 
in  vertical  movements,  and  also  for  lateral  ones  excepting  for  near- 
ness. The  cyclo-ductions  are  employed  essentially  as  supplementary 
to  horizontal  or  vertical  ductions  and  versions,  to  prevent  the  eyes  in 
these  movements  from  getting  out  of  binocular  alignment.  There 
are  cases  in  which  a  cyclo-duction  is  necessary  for  fusion  of  the 
images.  But  our  data. on  that  subject  is  very  limited  and  confined 
mostly  to  those  of  oblique  astigmatism  as  an  initial  cause. 

In  executing  a  duction  of  any  variety,  it  is  obvious  that  both  of 
the  muscles  of  the  duction  pair  for  that  direction  are  equally  en- 
gaged; for  if  they  were  not,  and  one  of  the  muscles  exercised  a 
stronger  traction  than  its  duction  mate,  the  eyes  would  not  be  bal- 
anced in  that  relative  position.  The  eye  upon  which  the  stronger 
traction  was  spent  would  turn  toward  the  muscle  exercising  it,  and 
the  other  eye  would  have  to  follow  it  to  preserve  fixation,  so  that 
this  action  would  balance  the  two,  making  tension  on  each  the  same 
or  converting  the  movement  into  a  version.  The  idea  of  one  muscle 
exercising  greater  duction  than  the  other  comes  from  confusing  the 
two  functions,  ductions  and  versions.  One  of  a  pair  of  duction  mus- 
cles may  be  acting  more  strongly  than  its  duction  associate,  but  in  that 
case,  a  simultaneous  version  or  a  muscular  imbalance  explains  it. 
It  is  not  a  duction  excess  of  one  muscle  above  the  other. 

Suppose  a  pair  of  orthophoric  eyes  are  fixing  an  object  in  a  di- 
rection that,  at  one  meter  from  the  eyes  is  lO  cm.  to  the  right  of  the 
vertical  medial  plane.  Then  the  right  eye  will  have  to  be  rotated  but 
7  cm.  rightward  from  its  primary  position,  while  the  left  eye  must  be 
rotated  13  cm.  rightward.  As  we  have  said  before,  this  is  a  "fanci- 
ful" supposition,  for  what  the  respective  eyes  rotate  to  fix  a  given 
object  depends  upon  their  position  at  starting.  If  the  two  eyes  had 
been  fixing  an  object  in  the  vertical  medial  (orbital)  plane,  at  i  meter 
distance,  to  change  from  that  position  to  a  point  10  cm.  rightward, 
both  would  turn  10  cm.  to  the  rightward  or  execute  a  version  of  10 A. 
The  adduction  of  the  two,  except  for  the  slight  amount  due  to 
obliquity,  would  be  continued  as  it  was  for  the  point  in  the  medial 
plane.    On  the  other  hand,  if  the  object  were  at  infinity  in  the  same 


MUSCLES   OF  THE   EYE  113 

oblique  direction,  the  visual  axes  would  be  parallel  in  that  oblique 
direction.  Bringing  the  object  up  to  the  meter  distance  would  merely 
cause  adduction  of  the  6A  required  for  that  distance.  Hence,  both 
the  version  and  the  duction,  considered  separately,  engage  the  mus- 
cles equally. 

Nerve  Control 

The  ductions  are  all  involuntary,  or  controlled  by  reflex  action. 
In  the  same  sense  as  it  used  to  be  said,  in  explaining  the  pressure  of 
fluids,  that  "nature  abhors  a  vacuum",  it  may  be  said,  in  this  connec- 
tion, that  "the  visual  sense  abhors  diplopia".  Hence,  the  various 
ductions  of  the  eyes  are  supplied  as  the  "guardian  angels"  to  protect 
the  visual  sense  from  experiencing  this  horror,  and  are  automatic 
in  their  operation  for  that  purpose.  But  a  reflex  action  is  exercised 
through  a  reflex  arch,  which  includes  sensory  nerves  to  convey  sen- 
sory messages,  a  ganglionic  center,  having  gray  matter  in  it,  to  re- 
ceive and  interpret  the  sensory  messages,  and  nervous  cells  to  gen- 
erate and  transmit  motor  impulse  to  the  muscles  required  to  act  to  re- 
lieve the  sensory  complaint.  While  under  the  supervision  of  the  gen- 
eral nervous  system,  locally  or  within  its  range,  it  has  complete  con- 
trol. 

There  must  then  be  a  nerve  center  for  the  stimulation  of  each  of 
the  varieties  of  duction,  or  directions  in  which  a  duction  pair  of  mus- 
cles act.  The  3d  nerve  ganglionic  center  has  been  located,  but  this 
center,  if  it  generates  all  3d  nerve  stimulations,  is  given  the  discre- 
tion of  a  small  brain,  and  perhaps  that  is  the  true  function  of  the 
ganglionic  nerve  centers.  They  are  little  brains,  set  apart  for  a  spe- 
cific function.  This  is  very  simple  when  applied  to  accommodation 
and  adduction,  for  both  are  stimulated  directly  by  the  3d  nerves.  It 
is  not  so  simple  when  abduction  is  to  be  accounted  for,  for  here  the 
6th  are  the  motor  nerves  and  they  must  have  a  different  motor-nerve 
center.  With  respect  to  the  vertical  ductions,  the  case  is  even  more 
complex,  for  sur-sum-duction  requires  stimulation  of  the  superior 
rectus  of  the  right  eye  with  the  inferior  rectus  of  the  left  eye;  and 
sum-sur-duction  engages  the  opposite  duction  pair;  while  each  pair 
acts  as  a  check  upon  the  other  pair,  thus  engaging  all  four  of  the 
muscles,  either  to  effect  or  check  the  rotation,  for  all  vertical  due- 


114 


MUSCLES   OF  THE   EYE 


tions.  As  all  four  of  these  muscles  are  stimulated  by  the  same  3d 
pair  of  nerves,  it  is  difficult  to  see,  unless  there  is  a  subordinate  nerve 
center  for  each,  how  these  ductions  can  be  effected  by  reflex  action. 


FIGURE   28. 

Binoculax  fixation  of  O,  in  plane  of  DO,  1  meter  from  the  eyes,  but  10  cm. 
to  right  of  M';  MM'  ipedial  orbital  plane;  MO  binocular  direction  of  O.  Right- 
ward  version  of  eyes,  lOA:  Convergence  of  eyes, -6 A;  FO  Visual  Axis  of  right 
eye;  F'O  Visual  Axis  of  left  eye.  Rotation  of  right  eye,  7A  from  D,  but  lOA 
from  M';  Rotation  of  left  eye,  13 A  from  S,  but  10  A  from  M'. 

However  that  is  not  a  question  for  the  optometrist  as  much  as  it  is 
for  the  physiologist,  who  has  the  laboratory  facilities  to  investi- 
gate it. 

But  there  can  be  no  doubt  these  ductions  are  exercised,  and  by 
reflex  action,  for  they  are  involuntary.  A  weak  prism  before  either 
eye,  base  down  or  base  up,  will  actively  engage  a  duction  pair,  and 
passively  or  as  checks,  the  opposite  pair.    Otherwise  there  would  be 


MUSCLES   OF   THE    EYE  115 

diplopia  at  once  when  the  prism  is  placed  as  above.  A  stronger 
prism  will  produce  the  diplopia  that  the  weaker  one  will  not,  showing 
that  the  muscles  are  able  to  neutralize  the  misplacement  of  the  images 
unless  it  is  of  such  an  amount  that  the  muscles  are  unable  to  neu- 
tralize the  misplacement.  The  cyclo-ductions  are  also  opponents,  and 
a  nerve  center  to  stimulate  each  pair  is  essential  to  their  involuntary 
action.  There  must  therefore  be  six  neuro-motor  centers  for  the  six 
different  directions  of  the  ductions  to  account  for  all  of  them,  unless 
one  grand  nerve-center  is  made  responsible  for  all  the  ductions.  Let 
our  brother  physiologists  wrestle  with  this  question. 

Duction  Tests 

Duction  tests  are  made  for  the  purpose  of  ascertaining  the  range 
of  duction  power  a  pair  of  duction  muscles  has,  measuring  in  meter- 
angles  or  prism-diopters.  Of  the  different  ductions,  that  of  adduc- 
tion is  naturally  the  greatest,  for  it  is  naturally  exercised  for  near 
vision,  and  because  the  function  of  accommodation  co-ordinates  with 
it  in  near  vision.  But,  in  stimilating  adduction  for  distance  by  plac- 
ing a  prism,  base  out,  before  either  eye,  the  accommodation  is  not 
called  into  action  by  the  nearness  of  the  object,  but  must  function  by 
itself.  Although  the  accommodation  and  adduction,  acting  together, 
may  have  a  near  point  that  calls  for  8  D.  of  accommodation  and  8- 
meter-angles  of  convergence,  which  convergence  requires  48A  of 
adduction,  in  a  distance  test  the  adductors  would  be  unable  to  over- 
come so  much,  or  fuse  the  images,  nor  could  the  accommodation  act 
so  strongly  but  for  the  co-ordination  of  adduction. 

When  a  near  target  is  moved  to  a  greater  distance  from  the  eyes, 
the  convergence  that  was  exercised  for  its  nearness  must  be  abated  or 
reduced.  The  mere  relaxation  of  the  adductors  is  not  Sufficient  to 
account  for  the  outward  movement  the  eyes  must  make.  Here  the 
muscles  to  check  over-convergence,  primarily  the  external  recti,  must 
contract  to  bring  the  visual  axes  more  nearly  to  the  parallel  positions, 
and  this  is  abduction.  At  the  same  time  the  accommodation  must  be 
reduced  for  a  greater  distance  of  the  object,  and  this  function  is 
exercised  by  the  fan  fibers  of  the  ciliary  muscles.  There  is  therefore 
co-ordination  of  this  ciliary  function  with  abduction,  unless  the  test 
of  abduction  is  made  for  a  distant  target  by  the  use  of  a  prism,  base 


116  MUSCLES   OF   THE    EYE 

in,  before  either  eye  or  both  eyes.  To  avoid  the  co-ordination  of  func- 
tions, a  duction  test  of  the  horizontal  muscles  is  best  made  with  a 
distant  target,  and  with  prisms.  Even  then  optical  defects  of  the 
eyes  that  abnormally  incite  accommodation  for  distance,  or  put  no 
demand  upon  it  for  near  vision,  must  be  taken  into  account.  In  fact, 
in  the  horizontal  meridian,  accommodation  and  convergence  are  so 
inter-related  that  the  refraction  must  first  be  corrected  before  any 
clear  idea  of  the  ductions  in  that  meridian  of  the  eyes  can  be  obtained 
with  certainty. 

Having  eliminated  the  accommodative  factor  (i)  by  employing 
a  distant  target,  and  (2)  by  correcting  the  refraction,  so  that  a  hyper- 
ope  will  not  accommodate  for  distance  and  a  myope  can  see  it,  we 
determine  the  highest  prism  power  the  different  duction  pairs  of 
muscles  are  able  to  overcome  by  proceeding  to  the  point  that  causes 
diplopia.  The  highest  prism  power  that  the  muscles  are  able  to  over- 
come, or  in  spite  of  which  they  fuse  the  images,  that  is  the  measure 
of  the  duction  power  in  that  direction,  or  with  that  pair  of  duction 
muscles. 

In  the  vertical  meridian,  a  prism  base  down  before  the  right 
eye,  or  base  up  before  the  left  eye,  tests  the  duction  power  of  the 
superior  rectus  of  the  right  with  the  inferior  rectus  of  the  left 
eye,  which  is  hyperpo-  or  sursum-duction.  If  these  muscles  fuse  the 
images  and  we  see  but  one  when  a  2°  prism  is  so  placed,  but  diplopia 
is  caused  by  a  3°  prism  in  the  same  position,  their  duction  power  is 
between  2°  and  3°.  To  test  the  opposite  pair,  which  is  hypoper-  or 
sumsur-duction,  the  prism  must  be  placed  base  down  before  the  left 
eye  or  base  up  before  the  right  eye.  Unless  there  is  a  vertical  im- 
balance, favoring  one  duction  pair  and  handicapping  the  other,  the 
duction  powers  should  be  equal.  But  if  there  is  4°  of  hyperpo-duc- 
tion  and  but  2°  of  hypoper-duction,  an  imbalance  favors  the  former 
1°  and  handicaps  the  latter  1°,  making  2°  for  both.  They  are  ex- 
pected to  be  equal,  for  there  is  no  natural  reason  for  one  being  de- 
veloped more  than  the  other. 

In  the  horizontal  meridian,  adduction  is  tested  by  prisms  base 
out  before  either  or  both  eyes.  Usually  the  duction  power  in  this  di- 
rection, exercised  by  the  internal  recti  muscles,  will  be  upwards  of 
20°  or  more,  as  this  is  a  much  used  function  and  developed  more 


MUSCLES   OF   THF.    EYE  117 

highly  than  any  other.  It  is  not  the  special  strength  of  these  muscles 
that  give  them  this  greater  duction  power,  for  a  duction  test  is  not  a 
test  of  the  muscular  strength  of  a  pair  of  muscles,  but  their  ability 
to  overcome  the  version  tendencies.  When  the  right  eye  is  turned 
leftward  or  inward,  there  is  a  natural  tendency  of  the  left  eye  to  turn 
in  the  same  direction ;  and  not  only  must  this  tendency  be  overcome, 
but  the  eye  actually  rotated  in  an  opposite  direction.  Hence,  even  ad- 
duction is  limited  in  range  by  this  "conflict  of  the  functions." 

Abduction  is  tested  by  prisms  base  in,  on  the  distant  target.  The 
prism  may  be  placed  over  one  eye  only  or  over  both  eyes,  their  bases 
being  of  course  in  opposite  directions.  The  duction  pair  of  muscles 
in  this  case,  are  the  external  recti.  In  the  versions  each  has  a  rotary 
power  of  about  lOoA,  but  when  used  together  as  a  duction  pair  they 
seldom  exceed  8 A,  or,  as  we  express  it  usually,  8°.  Nor  is  this  lim- 
ited duction  power  to  be  interpreted  as  "weakness"  of  these  muscles. 
They  have  an  ample  strength  for  rotating  the  eyes,  but  in  the  duc- 
tions  they  are  in  conflict  with  the  version  tendencies,  and  have  to 
appease  that  tendency  before  they  can  rotate  the  eyes  at  all  in  this 
manner.  The  only  natural  exercise  these  muscles  have  is  in  turning 
convergence  of  the  visual  axes  back  to  parallelism. 

If  the  target  is  at  20  ft.  it  should  be  of  sufficient  size  and  dis- 
tinctness to  be  plainly  visualized  by  both  eyes  at  that  distance.  A 
white  character  on  a  dull  black  background,  such  as  a  square  white 
card  2"  by  2"  is  suitable,  as  double  vision  of  it  would  be  distinctly 
marked.  By  the  use  of  a  red  disc  before  one  eye,  this  would  show, 
when  the  prism  caused  diplopia,  which  eye  saw  the  different  targets, 
white  and  red,  and  the  kind  of  diplopia.  More  elaborate  arrange- 
ments may  be  provided,  according  to  the  ingenuity  of  the  user.  The 
prisms  from  the  trial  case  answer  all  purposes,  although  the  use  of 
a  phorometer,  except  that  it  limits  the  prism  powers  that  may  be  em- 
ployed may  be  used  if  preferred.  There  must  be  means  of  making 
the  patient  understand  what  the  tests  mean,  if  his  or  her  co-operation 
with  you  is  to  be  obtained. 

Duction  tests  of  the  oblique  muscles,  acting  independently,  may 
be  made  with  a  pair  of  cylindrical  lenses  of  weak  power.  If  the  eyes 
are  both  emmetropic,  the  astigmatic  chart  at  20  ft,  appears  of  even 
color  and  distinctness  throughout.     Hence  a  pair  of  weak  plus  cyl- 


118 


MUSCLES   OF  THE   EYE 


inders,  -f  .50  cyls.  ax.  180,  will  blur  the  horizontal  lines,  leaving  the 
verticals  alone  distinct.  Rotation  of  the  cylinders  in  the  same  direc- 
tion causes  the  oblique  muscles  to  work  in  version  pairs,  to  preserve 
the  clear  vertical  lines  erect.  This  is  co-ordination  of  the  oblique 
muscles.    But  if  they  are  rotated  in  opposite  directions,  the  oblique 


FIGURE   29. 

Representing  corresponding  areas  of  retinae,  with  their  nervous  connection 
with  brain.  1  and  2,  right  and  left  brains;  3  and  4,  right  and  left  optic  tracts; 
5,  chiasm;  6  and  7,  right  and  left  optic  nerves;  D  and  D',  right  and  left  optic 
discs;  M  and  M',  right  and  left  maculae;  P  and  P*.  right  and  left  foveae;  a  and 
a,  a'  and  a',  b  and  b,  b'  and  b',  corresponding  quadrants  of  retinae. 

muscles  will  be  stimulated  in  duction  pairs  to  preserve  fusion  of  the 
clear  lines,  or  prevent  them  from  assuming  a  slant  or  cross  position. 
This  is  conjugation  of  the  obliques.  But  there  is  little  actual  rota- 
tion of  the  eyes  on  this  axis,  as  a  slight  rotation  tends  to  double  or 
cross  die  clear  lines. 

The  normal  duction  powers  of  the  several  pairs  of  duction  mus- 
cles are  about  as  follows : 

1.  Hyperpo-  or  sursum-duction,  2°   to   5°   or   A's. 

2.  Hypoper-  or  sumsur-duction,  2°   to  5°   or   A's. 

3.  Adduction,  without  training,  20  to  24  A's. 


MUSCLES   OF   THE    EYE  119 

4.  Abduction,  without  training,  5  to  8  A's. 

5.  Cyclo-ductions,  rotation  of  2°  to  5°  of  .50  cyls. 

The  normal  ratio  of  adduction  to  abduction  is  about  3  to  i, 
for  the  adductors  are  in  constant  use  to  rotate  the  eyes  to  conver- 
gence for  near  vision,  while  the  abductors  are  only  normally  em- 
ployed in  the  negative  capacity  of  bringing  the  converged  visual  axes 
back  to  parallel  positions.  If  the  abduction  is  ever  found  equal  to 
adduction,  it  indicates  an  imbalance  favorable  to  the  abductors  and 
handicapping  the  adductors  to  the  same  extent.  But  counting  the 
imbalance  against  the  pair  favored  by  it,  and  with  the  pair  handi- 
capped, the  ratio  is  then  about  as  3  to  i. 

•It  is  seen  therefore  that  "imbalances"  of  the  duction  pairs  of 
muscles  may  give  a  pair  the  appearance  of  greater  power  than  be- 
longs to  them,  and  the  opposite  pair  an  appearance  of  weakness  that 
does  not  pertain  to  them.  We  therefore  have  this  as  a  new  factor 
to  deal  with,  to  discount  the  apparent  superiority  of  one  duction  pair 
over  another  and  to  add  to  the  power  of  the  apparently  weak  pair. 
As  this  factor  enters  the  field  in  all  duction  tests  it  must  be  taken  into 
account,  which  leads  to  the  discussion  of  the  muscular  imbalances, 
or  heterophoria. 

Reflex  Influences 

As  the  ductions  are  involuntary,  or  operated  by  reflex  action,  the 
sensory  warning  that  the  images  tend  to  separate,  producing  diplopia, 
is  conveyed  over  sensory  nerves  to  the  ganglionic  nerve-center  hav- 
ing control  over  the  muscles  that  maintain  fixation ;  the  message  or 
warning  is  received  and  interpreted  by  the  ganglionic  nerv^e-center ; 
the  generation  of  motor  stimulus  at  that  center,  and  its  direction 
and  transmission  to  the  appropriate  muscle  or  muscles  follows ;  and 
the  response  of  the  muscles  to  the  stimulus,  and  thus  keeping  the 
eyes  and  vision  out  of  trouble,  give  the  reflex  arches  having  control 
over  the  different  activities  plenty  to  do.  Like  the  telephone  wires, 
the  sensory  and  motor  nerves  are  always  "humming"  with  messages, 
or  in  a  perpetual  state  of  excitation  and  action,  even  while  one  is  en- 
joying the  quiet  pastime  of  a  stroll  down  the  street,  in  the  park,  or 
over  a  country  road.  For  the  scene  is  constantly  shifting  and  the 
point  of  fixation  is  constantly  changing,  keeping  both  the  versions 


120  MUSCLES   OF  THE   EYE 

and  the  ductions  perpetually  in  a  state  of  activity.  And  yet  so  per- 
fect are  the  physiological  provisions  for  these  sensory  and  motor 
nerve  activities,  and  for  their  control  of  the  ocular  muscles,  that  all 
of  these  functions  operate  without  even  diverting  our  attention  from 
"what"  we  see,  instead  of  engaging  it  upon  "how"  we  see  it.  We  do 
not  have  to  think  about  what  our  eyes  or  their  muscles  and  nerves  are 
doing. 

While  sauntering  along  we  give  ourselves  up  to  dreams  of  other 
things,  perhaps  far  from  our  immediate  environment,  trusting  to  the 
spontaneity  of  these  reflexes  to  take  in  all  the  sights  on  the  way  and 
to  be  attracted  by  anything  unusual  or  out  of  the  ordinary.  For  the 
eyes  alone  and  their  activities,  six  of  the  twelve  pairs  of  cranial 
nerves  are  directly  employed,  and  we  must  control  the  pupil  and 
regulate  the  accommodation  by  the  same  means.  The  exercise  of  the 
versions  and  ductions  are  an  important  part  of  it  all,  but  not  the 
whole  of  it.  The  other  six  pairs  of  cranial  nerves  are  not,  in  the 
meantime,  idle.  We  may  be  engaged  in  smoking  a  cigar  or  in  eating 
an  apple  or  orange  as  we  saunter  along ;  and  the  smell  of  the  flowers 
or  of  the  new-mown  hay,  the  songs  of  the  birds  or  lowing  of  cattle 
in  the  field  engage  the  other  special  senses  simultaneously.  We  may 
have  a  companion  in  our  stroll,  and  engage  in  conversation  as  we 
walk  along,  but  we  note  every  little  rise  of  ground  we  must  step  up 
to,  and  how  to  avoid  obstacles  by  lifting  our  feet  over  them.  These 
muscular  movements  engage  other  than  the  ocular  muscles,  and 
other  reflex  arches  provide  the  means  for  involuntary  control.  Along 
the  spinal  column  there  is  a  row  of  ganglionic  centers  by  which  par- 
ticular reflexes  are  controlled.  Our  lungs  are  kept  busy  providing  the 
blood  with  oxygen,  the  heart  in  sending  blood  to  the  lungs  for  puri- 
fication, and  to  the  body  for  nourishment,  and  to  the  brain  for  mental 
purposes ;  and  digestive  processes  go  right  on  amidst  it  all,  and  largely 
by  reflex  action  all  of  these  activities  are  kept  up. 

Whatever  we  may  be  talking  about  or  thinking  about,  the  re- 
flexes proceed  to  function  right  along  their  special  lines.  But 
always  there  should  be  a  sort  of  sixth  sense  that  keeps  us  alert  to  our 
surroundings,  and  especially  the  approach  of  danger.  With  all  of 
these  reflexes  in  active  operation,  there  is  little  wonder  that  they 
should  exercise  mutual  influences  over  each  other.    All  of  the  reflex 


MUSCLES   OF   THE    EYE  121 

activities  of  the  body,  including  those  of  the  eyes,  are  bound  into 
sympathetic  relationships,  and  the  general  and  sympathetic  nervous 
systems  are  the  electric  cables  that  bring  them  into  communication 
with  each  other.  Abnormality  in  the  functioning  of  the  eyes  may 
thus  have  a  direct  effect  upon  the  digestion,  or  vice  versa.  Certainly 
the  muscular  functions  of  the  eyes  have  a  powerful  influence,  first 
upon  each  other  and  second  upon  the  organic  functioning  of  every 
organ.  Compared  with  the  steamship,  the  human  anatomy  and 
physiology,  so  largely  controlled  by  automatic  reflexes,  is  a  compli- 
cated mechanism,  more  scientifically  designed  and  more  perfect  in 
its  functioning,  but  subject  also  to  greater  complexities  of  derange- 
ment. It  is  expecting  too  much  of  any  class  of  professionals  to  as- 
sume that  a  single  profession  can  be  capable  of  dealing  with  all  of 
these  derangements  and  the  means  of  relieving  them.  To  be  thor- 
oughly competent  in  any  field  necessitates  the  relinquishment,  to  other 
professions,  of  supervision  over  special  ones.  The  field  of  optometry 
is  indicated  by  the  title  of  this  book. 


122  MUSCLES   OF  THE    EYE 

CHAPTER  VIII 
Muscular  Imbalance 

Any  abnormality  in  the  structure  or  functional  action  of  a 
muscle  may  be  termed  a  muscular  anomaly.  The  ocular  muscles — 
iris,  ciliary  or  extrinsic  muscles — are  subject  to  such  anomalies.  A 
muscle  that  is  normal  in  its  action,  may  function  abnormally  because 
an  abnormal  structure  interferes  with  its  normal  action,  or  it  may 
function  abnormally  on  its  own  account.  In  optometry  we  give  little 
attention  to  the  anomalies  of  individual  muscles,  as  we  have  no  direct 
optical  means  of  correcting  them.  With  respect  to  the  extrinsic 
muscles,  our  attention  is  centered  upon  their  binocular  poise  or 
balance,  since  the  neutralization  of  a  defect  of  that  kind  may  be 
effected  by  optical  means.  Without  attributing  a  muscular  imbalance 
to  a  specific  muscle,  or  to  a  binocular  pair  of  muscles,  the  fact  is 
made  apparent  that  there  is  some  structural  or  functional  inequality 
of  different  binocular  pairs  that  give  the  eyes  a  tendency  to  assume 
abnormal  positions,  or  relative  positions,  and  so  disturb  the  normal 
functioning  of  the  muscles,  both  for  versions  and  for  ductions. 

We  may  attribute  the  anomaly  to  the  structural  length  of  a 
muscle  or  pair  of  muscles,  or  to  their  attachment  to  the  eye-balls,  but 
it  is,  in  that  case,  their  relative,  not  their  absolute,  length  or  attach- 
ments ;  and  whether  it  pertains  to  a  single  muscle  or  to  a  single  pair 
of  muscles,  or  to  both  muscles  or  both  pairs,  is  immaterial.  It  is  their 
relationship,  or  relativity,  in  length  or  attachment,  that  accounts  for 
the  anomaly,  if  it  is  a  structural  defect.  The  extrinsic  ocular  mus- 
cles are  so  related  binocularly  in  effecting  the  versions  and  ductions, 
that  a  defect  in  one  of  the  muscles,  structural  or  functional,  involves 
a  pair  of  muscles ;  and  a  defect  of  one  binocular  pair  of  muscles  in- 
volves the  opposite  pair,  for  it  throws  an  abnormal  functional  bur- 
den upon  them.  It  is  even  possible,  though  not  probable,  that  the 
anomaly  of  one  muscle  may  be  compensated  for  by  the  opposite 
anomaly  of  its  binocular  associate,  thus  balancing  the  pair,  although 
both  muscles  are  abnormal,  either  in  structure  or  function.  To  the 
surgeon  who  designs  to  operate  for  the  correction  of  a  defect,  this 
may  be  an  important  question,  but  it  is  much  less  so  to  the  optome- 
trist. 


MUSCLES   OF  THE   EYE  123 

The  Homophorias 

A  tendency  of  the  eyes  to  deviate  abnormally,  but  equally,  and 
in  the  same  direction,  as  to  the  right  or  left,  up  or  down,  is  termed 
a  homophoria.  As  such  a  tendency  can  be  neutralized  by  a  slight 
turning  of  the  head,  thus  allowing  the  eyes  to  have  their  way,  little 
attention  is  paid  to  them.  If  prisms  were  prescribed  for  a  homo- 
phoria, they  would  be  of  equal  value  before  each  eye  and  their 
bases  would  be  in  the  same  direction,  the  apex  of  each  pointing  in 
the  direction  of  the  tendency  of  the  eyes  to  deviate.  The  classifica- 
tion of  these  anomalies  is  given  in  a  previous  section.  Muscularly 
they  are  neutralized  by  a  version  in  the  direction  opposite  to  the 
tendency.  But,  as  this  action  would  have  to  be  maintained,  it 
would  become  tiresome  to  the  muscles.  Therefore  the  head  is 
allowed  to  turn  in  the  opposite  direction  so  as  to  allow  the  eyes 
to  assume  the  position  most  comfortable  to  the  muscles,  the  same 
as  we  turn  the  head  to  look  at  an  object  in  an  oblique  direction 
rather  than  rotate  the  eyes  in  their  orbits  and  maintain  them  there. 
The  movements  of  the  head  and  body  co-ordinate  with  the  versions 
and  relieve  the  version  pairs  of  muscles  of  much  of  the  work  they 
would  otherwise  be  called  upon  to  exercise,  although  their  range  of 
movement  is  much  wider  or  more  ample  in  extent  than  the  ductions. 
A  homophoria,  although  it  is  a  muscular  anomaly,  is  not  a  muscular 
imbalance. 

The  Heterophorias 

If  the  relative  lengths  or  attachments  of  binocular  pairs  of 
muscles  is  such  as  to  give  the  eyes  a  tendency  to  turn  out  of  normal 
alignment  for  the  fixation  of  an  objective  point,  or  if  a  functional 
weakness  of  a  single  muscle,  or  of  a  binocular  pair  of  muscles, 
tends  to  handicap  or  derange  normal  binocular  fixation,  this  defect 
or  abnormality  is  termed  a  heterophoria.  But  if  the  effect  of  the 
tendency  is  to  actually  turn  the  eyes  out  of  normal  binocular  align- 
ment, so  that  binocular  fixation  cannot  be  maintained,  fusion  of  the 
images  becomes  impossible,  and  diplopia  or  double-vision  results, 
it  is  termed  heterotropia  or  strabismus.  The  muscular  defects  are 
sometimes  compared  as  latent  and  manifest  strabismus — ^that  is, 
heterophoria  as  latent  heterotropia,  or  heterotropia  as  manifest  hetero- 


124  MUSCLES   OF   THE    EYE 

phoria.  In  the  'phorias,  as  they  are  briefly  referred  to,  the  function- 
ing of  the  muscles  preserves  fusion  of  the  images  in  spite  of  the 
abnormal  tendencies;  but  in  the  'tropias,  fusion  of  the  images  and 
single  vision  are  sacrificed. 

As  in  the  definitions  of  the  different  refractive  states  of  the 
eyes,  or  the  relativity  of  the  refraction  to  the  axial  depth  of  the  eyC; 
we  must  have  a  standard  or  datum  to  determine  the  normal  from  the 
abnormal  condition.  With  the  muscles  this  datum  is  parallelism  of 
the  visual  axes,  as  required  for  the  binocular  fixation  of  a  distant 
object.  Its  direction  from  the  eyes,  or  its  binocular  direction,  is 
not  a  factor  in  the  maintenance  of  binocular  fixation  and  fusion  of 
the  images,  as  that  is  provided  for  in  the  binocular  versions  of  the 
eyes.  Its  distance,  in  connection  with  the  binocular  poise  or  bal- 
ance of  the  muscles,  provide  the  only  direct  factors  involved.  For 
a  distant  object,  one  at  infinity,  whatever  its  direction,  binocular 
fixation  requires  that  the  visual  axes  be  parallel.  If  such  binocular 
fixation  is  obtained  and  maintained  without  special  tension  upon  a 
duction  pair  of  muscles,  the  eyes  are  of  normal  balance,  or  ortlio- 
phoric.  But  if  special  tension  on  a  duction  pair  of  muscles  is  required 
to  parallel  the  visual  axes,  then  the  eyes  are  muscularly  unbal- 
anced, or  heterophoric. 

Direction  of  Imbalance 

The  classification  of  the  heterophoric  is  based  upon  the  direc- 
tion of  the  tendency  of  the  eyes  to  deviate  from  parallelism  of  the 
visual  axes,  though  perhaps  more  clearly  defined  by  the  duction 
that  is  necessary  to  maintain  parallelism,  such  duction  being  natur- 
ally in  the  opposite  directions  from  the  tendency  of  the  eyes  to 
deviate.  These  directions  are,  primarily,  vertical,  horizontal  and 
circular  or  cyclical,  although  the  latter  is  essentially  subsidiary. 
These  classes  are  as  follows : 

Vertical — 

I.     Hyperpo-phoria  (R.  hyperphoria),  a  tendency  of  the 

eyes,  or  their  visual  axes,  to  assume  different  horizontal  levels, 

the  right  eye  tending  to  rotate  to  a  higher  position  than  the  left 

eye. 


MUSCLES   OF   THE    EYE  125 

This  tendency  is  neutralized  muscularly  by  a  hypoper-  or 
sumsur-duction,  primarily  by  the  inferior  rectus  of  the  right 
eye  with  the  superior  rectus  of  the  left  eye, 

2.  Hypoper-phoria  (L.  hyperphoria),  a  tendency  of  the 
eyes,  or  of  their  visual  axes,  to  assume  different  horizontal  levels, 
the  right  eye  tending  to  rotate  to  a  lower  position  than  the  left. 

This  tendency  is  neutralized  muscularly  by  a  hyperpo-  or 
sursum-duction,  primarily  engaging  the  superior  rectus  of  the 
right  eye  with  the  inferior  rectus  of  the  left  eye. 
Horizontal — 

3.  Esophoria,  a  tendency  of  the  eyes,  or  of  their  visual 
axes,  to  assume  a  convergent  position,  or  to  turn  toward  each 
other  and  cross  at  some  point. 

This  tendency  is  neutralized  muscularly  by  abduction,  an 
action  that  primarily  engages  the  external  recti  muscles,  and 
must  be  constant  to  hold  the  visual  axes  parallel. 

4.  Exophoria,  a  tendency  of  the  eyes,  or  of  their  visual 
axes,  to  diverge,  or  rotate  outward  from  the  normal  parallel 
position  for  distant  vision. 

This  tendency  is  neutralized  muscularly  by  adduction,  an 
action  that  primarily  engages  the  internal  recti  muscles,  and 
must  be  maintained  to  parallel  the  visual  axes  for  distant  vision, 
and  is  increased  for  near  vision. 
Cyclical — 

5.  Supra-cyclo-phoria,  a  tendency  of  the  eyes  to  assume 
oblique  meridianal  positions,  so  that  the  normal  vertical  meri- 
dians would  be  converged  above,  or  upward. 

This  tendency  is  neutralized  muscularly  by  infra-cyclo- 
duction,  which  engages  the  inferior  oblique  muscles. 

6.  Infra-cyclo-phoria,  a  tendency  of  the  eyes  to  assume 
oblique  meridianal  positions,  causing  the  normal  vertical  meri- 
dians to  converge  below,  or  downward. 

This  tendency  is  neutralized  muscularly  by  a  supra-cyclo- 
duction,  which  engages  the  superior  oblique  muscles. 
The  terms  or  prefixes  "hyperpo"  and  "hypoper,"  which  are  ab- 
breviations of  the  compounds,  "hyper-hypo"  and  "hypo-hyper"  are 
adopted  to  avoid  the  misleading  terms,  right  and  left  hyperphoria, 


126  MUSCLES   OF  THE   EYE 

which  give  the  impression  that  the  condition  intended  to  be  repre- 
sented by  them  pertains  to  the  eye  designated,  right  or  left,  or  to 
its  muscles,  and  that  the  other  eye  is  not  involved.  They  fail  to  make 
clear  the  seemingly  obvious  fact  that,  if  one  eye  tends  to  turn  higher 
than  the  other,  the  other  eye  tends  equally  to  turn  lov^^er  than  it,  and 
that  it  is  a  binocular,  not  a  monocular,  condition.  Therefore  the 
term  "right  hyperphoria"  is  interpreted  to  mean  that  the  right  eye 
tends  to  turn  upward,  or  "left  hyperphoria"  that  the  left  eye  tends 
to  turn  upward,  so  that  we  must  also  have  the  terms  "right  hypo- 
phoria" and  "left  hypophoria"  to  indicate  the  opposite  tendency  of 
each  eye  downward.  It  would  be  equally  appropriate  to  have  the 
terms  right  and  left  exophoria,  and  right  and  left  esophoria,  the 
duality  of  which  is  apparent. 

It  is  the  misleading  character  of  these  expressions  that  causes 
or  validates  the  terms  "supra-duction"  of  the  right  or  left  eye,  and 
"infra-duction"  of  either  by  itself.  According  to  these  terms,  the 
effect  of  a  prism,  base  down,  before  the  right  eye,  is  to  test  the 
supra-duction  of  that  eye,  or  the  strength  of  its  superior  rectus  mus- 
cle ;  while  the  same  prism,  base  up,  before  the  left  eye,  is  a  test  of  its 
infra-duction,  or  the  strength  of  its  inferior  rectus  muscle.  But 
either  of  the  positions  of  the  prism  is  a  test  of  the  "Supra-infra- 
duction"  of  its  muscles  in  combined  binocular  action,  and  not  of 
either  of  them  alone.  If  the  prism,  base  down  before  the  right  eye 
alone,  causes  its  superior  rectus  to  contract  to  turn  that  eye  upward, 
it  also  causes  an  equal  contraction  of  the  inferior  rectus  of  the  left 
eye  to  prevent  it  from  rotating  upward  with  the  right,  or  to  hold  it 
in  position  while  the  right  eye  is  rotated  upward,  for  the  left  eye, 
under  its  version  impulses,  will  naturally  tend  to  rotate  upward  wi^ 
the  right  eye.  But,  to  maintain  binocular  fixation  and  fusion,  it  can- 
not be  allowed  to  do  so. 

Motor-Nerve  Controls 

The*  relationship  between  certain  of  the  muscular  functions  is 
usually  explained  on  the  basis  c|^  a  direct  connection  between  or  prox- 
imity to  each  other  of  special  nerve  centers.  While  that  explanation 
may  account  in  part  for  some  of  the  associations,  it  is  far  from  being 
completely  satisfactory.    There  is  no  such  connection  in  the  associa- 


MUSCLES   OF  THE   EYE  127 

tion  of  the  version  pairs  of  muscles.  To  execute  a  rightward  ver- 
sion of  the  eyes,  the  externa)  rectus  of  the  right  eye  is  associated 
with  the  internal  rectus  of  the  left  eye.  The  first  is  innervated  by 
the  6th  nerves,  while  the  second  is  innervated  by  the  3d  nerves. 
There  has  never  been  set  up  a  claim  that  these  two  different  pairs  of 
motor-nerves»had  a  common  center,  for  they  operate  muscles  that,  in 
the  ductions,  are  antagonistic  to  each  other.  A  leftward  version  in- 
volves the  same  problem  or  enigma  of  muscular  or  motor-nerve  as- 
sociation. And  yet  the  association  of  the  muscles  in  version  pairs  is 
the  strongest  association  there  is. 

To  rotate  the  right  eye  upward,  or  to  rotate  the  right  eye  down- 
ward, is  a  very  easy  muscular  action,  provided  the  left  eye  is  per- 
mitted to  rotate  the  same  amount  in  the  same  direction,  and  if  you  do 
not  permit  it,  it  will  rotate  upward  or  downward  with  the  right  eye 
without  your  permission.  Wherever  the  right  eye  turns,  voluntarily, 
the  left  eye  involuntarily  goes  along  with  it,  or  is  strongly  inclined  so 
to  do,  and  it  takes  a  real  force  to  stop  it.  It  is  a  primordial  associa- 
tion, and  may  date  back  to  a  time  when  we  had  but  one  eye  to  rotate, 
for  the  two  naturally  rotate  together  as  though  they  were  one.  We 
don't  have  to  teach  the  eyes  to  function  in  this  manner,  although  there 
is  no  common  nerve  center  to  account  for  the  association  of  external 
with  an  internal  rectus  muscle.  Nerve  centers  do  not  altogether  ex- 
plain the  association,  but  the  unexplained  association  functions  as 
easily  as  if  we  had  the  most  plausible  explanation  of  it.  So  much 
for  that. 

The  two  superior  recti  muscles  rotate  the  eyes  together  upward. 
We  cannot,  with  any  consistency  say  that  it  is  because  they  are  both 
innervated  by  the  3d  cranial  nerves,  for  this  is  merely  a  hyper-  or 
supra-version,  and  if  it  is  not  necessary  horizontally,  for  a  rightward 
or  leftward  version,  why  should  it  be  vertically  for  an  upward  ver- 
sion? But  as  the  3d  nerves  also  supply  the  inferior  recti  muscles, 
why  should  not  the  motor  stimulus  sent  to  the  superiors  to^cause  an 
upward  version  also  reach  the  inferiors  and  counteract  the  upward 
by  a  downward  version?  A  nerve  center  must  be  given  greater  "dis- 
cretion" in  deciding  what  to  do,  under  specific  circumstances,  than 
has  ever  been  given  to  it,  or  than  is  given  it  by  the  stereotyped  ex- 
planations of  these  associations.     We  must  credit  the  nerve  centers 


128  MUSCLES   OF   THE    EYE 

with  greater  powers  than  is  usually  assigned  them,  to  account  for 
their  effecting  associate  actions  of  the  muscles.  A  good  deal  of  this 
is  due  to  habit  or  heredity,  rather  than  to  the  clustering  of  nerve  cen- 
ters about  a  locality.  Nerve  centers  are  the  product  of  a  necessity, 
evolved  by  the  necessity,  rather  than  procreated  to  meet  a  require- 
ment that  has  not  as  yet  manifested  itself,  and  hence  we  may  expect 
that  they  will  continue  to  be  supplied  as  necessity  demands  them.  Our 
anatomy  and  physiology  is  not  yet  finished.  It  continues  to  be  de- 
manded anew,  and  evolved  by  necessity.  Improvements  are  not 
confined  to  automobiles  and  airships.  Men  are  needed  to  evolve  them 
and  men  with  improved  physical  and  mental  powers.  We  pick  up 
the  old  powers  by  heredity;  necessity  forces  the  new  ones  upon  us. 
In  a  hyperpo-  or  hypoper-phoria,  the  muscles  primarily  involved  are 
a  superior  rectus  of  one  eye  with  the  inferior  rectus  of  the  other, 
both  innervated  by  the  3d  nerves,  but  notwithstanding  that  fact,  the 
duction  power  is  very  limited,  not  usually  above  2 A,  although  in 
version  pairs  they  easily  negotiate  40  to  50 A. 

Inductive  Influence 

The  muscular  functions  of  the  eyes  are  not  isolated,  either 
monocularly  or  binocularly.  They  inductively  influence  the  same 
functional  action  in  each  other  monocularly  for  the  intrinsic  func- 
tions, accommodation  and  pupillary  control ;  corresponding  binocu- 
lar action  of  the  extrinsic  muscular  functions ;  and  co-ordination  of 
the  intrinsic  with  the  extrinsic  muscular  functions.  To  indicate 
these  functional  relationships  we  use  the  word  "induction"  in  the 
same  sense  as  it  is  employed  in  electricity,  meaning  the  excitation  of 
a  passive  muscular  agent  by  an  active  one,  although  there  may  exist 
no  direct  cause  for  the  action  of  the  agent  that  is  thus  excited  or  in- 
cited to  action.  It  is  to  this  class  of  activities  that  the  term  "physio- 
logical" may  be  applied.  The  action  is  due  to  the  physiological  in- 
fluence of  another  agent,  and  not  to  any  structural  defect  or  objective 
cause.  The  eyes  are  not  individual  in  these  respects,  but  a  pair,  a 
team,  and  work  together. 

Intrinsic  Inductions 

Each  eye  is  provided  with  the  means  of  controlling  its  own 
accommodative  action  and  of  exercising  pupillary  control  over  its 


MUSCLES   OF   THE    EYE  129 

own  pupil.  The  demands  for  such  actions  may  be  different  in  the 
two  eyes,  and  often  are  different,  as  in  anisometropia.  But  never- 
theless there  is  invariably  a  tendency  of  either  of  these  functions  to 
act  together  in  the  two  eyes,  and  of  both  to  be  exercised  when  one  of 
them  is  excited.  Covering  one  eye  with  an  opaque  screen  while 
bringing  an  influence  to  bear  upon  the  uncovered  eye  that  causes 
contraction  or  expansion  of  its  pupil,  the  covered  eye,  behind  its 
screen  responds  to  the  influence  in  a  less  degree,  but  shows  a  sym- 
pathetic association  with  its  binocular  mate.  When  the  accommoda- 
tion of  one  of  the  eyes  is  excited,  there  is  a  responsive  action  in  the 
other  eye,  though  it  may  not  be  brought  under  the  same  direct  in- 
fluence. 

In  using  the  subjective  method  known  as  the  "fogging  system" 
for  the  abatement  of  the  accommodation,  more  complete  ciliary  re- 
laxation is  obtained  by  applying  it  to  the  two  eyes  together  than  by 
applying  it  to  either  eye  alone.  This  may  be  done  by  using  corre- 
sponding lenses  for  the  two  eyes,  lenses  of  the  same  power,  up  to 
the  point  where  subjective  vision,  or  its  impairment,  indicates  that 
both  eyes  are  in  the  fog,  although  one  may  be  deeper  in  it  than  the 
other  with  the  equal  lenses.  At  this  point,  or  in  reducing  the  fog, 
since  the  accommodation  has  been  quieted  in  both,  at  least  the  mani- 
fest accommodation,  one  eye  may  be  covered  while  making  the  re- 
ductions one  eye  at  a  time.  Even  then,  after  each  has  been  reduced 
to  the  required  value  for  the  best  distant  vision,  it  may  be  found  that 
together  they  will  submit  cheerfully  to  an  increase  in  the  plus  correc- 
tions found.  Accommodative  relaxation  is  thus  shown  to  be  most 
complete  when  the  eyes  function  binocularly. 

Extrinsic  Combinations 

The  eyes  are  seldom  turned  on  primary  rotary  axes,  as  the  ver- 
tical, the  horizontal  or  the  anterio-posterior  axes,  but  on  some  com- 
posite axes  extending  in  the  same  direction  through  the  rotary  cen- 
ters. The  direction  of  rotation  may  be  up,  to  the  left,  and  inward; 
or  down,  to  the  right  and  outward.  These  are  normal  rotations,  rota- 
tions that  are  normal  for  orthophoria.  They  are  not  executed  by 
any  single  pair  of  binocular  muscles,  but  by  pairs  of  muscles  in  vari- 
ous combinations.    For  the  first  movement  above,  up,  to  the  left  and 


130  MUSCLES   OF   THE    EYE 

inward,  there  is  an  upward  or  hyper-version,  with  a  leftward  or  sin- 
estra-version ;  and  these  are  combined  with  an  adduction  of  greater 
or  less  amount,  depending  upon  the  distance,  or  rather  upon  the 
nearness,  of  the  object  fixed.  If  the  objective  point  of  fixation  is 
located,  or  its  position  relative  to  the  medial  planes,  and  distance 
from  the  correction  plane,  is  fixed,  the  amount  of  each  action  may 
be  stated  in  prism-diopters.  For  the  versions  specified,  version 
pairs  of  muscles  are  combined ;  for  the  duction  required,  a  duction 
pair  of  muscles  are  engaged ;  and  there  are  also  the  opposite  pairs 
of  muscles  acting  as  checks  to  the  movement.  But  the  net  result  of 
all  of  these  is  but  the  rotation  of  each  eye  upon  a  single  axis  of 
rotation.  The  visual  axes  are  to  be  placed  in  a  fixed  position,  and 
the  direct  rotation  that  fixes  them  in  that  position  is  made,  such 
muscles  being  employed  as  may  be  required  to  effect  the  movements, 
monocular  and  binocular. 

While  the  direct  action  of  the  oblique  muscles  is  to  rotate  the 
eyes  on  their  anterio-posterior  axes,  either  in  a  version  or  a  duction, 
their  functioning  in  that  manner  is  for  but  slight  amounts,  and  sel- 
dom required.  But  they  are  employed  chiefly  for  steadying  the  eyes 
in  their  rectilinear  versions  and  ductions,  thus  keeping  the  retinas 
"square"  to  the  image-forming  incident  light  from  the  objective  field 
of  vision.  In  doing  this  they  co-ordinate,  but  to  a  small  extent,  with 
the  recti  muscles  in  making  rectilinear  rotations  of  the  eyes,  and  pre- 
venting the  abnormal  "roUing"  of  the  eyes  in  making  such  move- 
ments, especially  for  oblique  directions  of  the  object.  Hence,  to 
effect  the  required  versions  and  ductions,  and  hold  the  eyes  in  due 
receiving  positions,  the  entire  extrinsic  system  of  muscles  is  in  a 
state  of  perpetual  activity,  even  in  normal  eyes  that  are  in  normal 
muscular  balance.  But  an  imbalance  of  the  muscles  or  other  abnor- 
mality such  as  a  homophoria,  adds  a  further  element  of  tension  to 
one  or  more  pairs  of  muscles.  Between  all  of  these  demands,  the 
ocular  muscles  engage  and  consume  an  enormous  amount  of  motor- 
nerve  energy — an  amount  that  sums  up  at  least  equal  to  that  of  the 
more  powerful  muscles  of  the  legs,  arms  or  body,  except,  perhaps, 
for  manual  labor ;  or  the  muscles  of  the  tongue  and  other  organs  of 
speech  by  the  salesman,  politician  or  public  speaker.  They  are  more 
constant  in  their  activities  than  any  of  these. 


MUSCLES   OF  THE   EYE  131 

Extrinsics  With  Intrinsics 

But  the  inductive  influences  of  the  extrinsic  upon  the  intrinsic 
muscles,  and  vice  versa,  are  of  very  much  greater  importance.  Partly 
on  account  of  the  proximity  of  ganglionic  nerve-centers  to  each 
other;  partly  from  the  distribution  of  motor-nerves  from  the  same 
or  different  nerve-centers  to  the  same  or  different  muscles ;  partly 
from  the  identity  of  objective  demand,  or  necessity  of  co-ordina- 
tion, and  the  habit  or  practice  of  acting  together,  entailing  an  hered- 
itary tendency  to  co-ordination,  becoming  fettered  by  necessity,  there 
is  built  up  certain  associations  between  extrinsic  and  intrinsic  mus- 
cular functions  that  must  be  taken  into  account  in  dealing  optically 
with  either. 

The  most  generally  recognized  association  of  this  kind  is  that 
between  the  functions  by  which  the  eyes  are  muscularly  adjusted  to 
changes  from  far  to  near  vision,  or  from  near  to  far  vision.  The 
association  is  between  the  horizontal  extrinsics  (the  internal  and 
external  recti  in  duction  pairs)  and  the  ciliary  muscles;  and  to  a 
less  degree,  the  iris.  This  association  is  usually  referred  to  as  that 
of  "Accommodation  and  Convergence",  each  of  which  is  the  "effect" 
of  a  functional  action  rather  than  the  function  itself.  Convergence  is 
a  physical  or  static  term,  without  reference  to  the  functional  action 
that  brings  it  about.  The  functional  action  is  Adduction,  the  pri- 
mary muscular  factors  of  which  are  the  internal  recti,  with  the  ex- 
ternal recti  acting  as  checks  to  over-convergence  of  the  eyes.  The 
term  "adduction"  is  better  suited  to  represent  this  factor  of  the 
association,  for  it  may  be  exceedingly  active  without  effecting  any 
convergence  whatever. 

The  accommodation  is  not  a  one-direction  action  merely,  but 
as  the  term  indicates,  it  is  the  functional  power  of  adapting  the  re- 
fraction of  the  eye  to  objects  at  greater,  as  well  as  at  less,  distances. 
If  the  adaptation  of  its  refraction  to  a  less  distance,  by  increasing 
the  convexity  of  the  crystalline  lens,  is  positive  accommodation,  its 
adaptation  to  a  greater  from  a  less  distance,  by  reducing  the  con- 
vexity of  the  crystalline  lens,  is  negative  accommodation.  There  is 
every  reason  to  believe  that  the  eye  functions  muscularly,  but  alter- 
nately, in  both  directions,  and  that  passivity  occurs  only  when  an 
emmetropic  eye  views  distant  objects,  or  a  myopic  eye  is  engaged  in 


132  MUSCLES   OF  THE   EYE 

seeing  an  object  at  its  far  point.  The  purpose  of  a  distant  correc- 
tion of  the  eye  with  lenses  is  to  reduce  accommodation  to  this  state, 
passivity  for  distant  vision,  positive  accommodation  for  a  nearer 
object,  negative  accommodation  for  the  neutraHzation  of  positive ; 
but  in  extreme  youth  this  may  function  slightly  to  neutralize  myopia 
of  a  low^  degree. 

The  belief  in  this  functional  power  is  ba.sed  upon  the  apparent 
muscular  provisions  for  it  in  the  variety  of  arrangement  of  the  mus- 
cular fibers  of  the  ciliary  muscle,  the  pliable  character  of  the  triangu- 
lar ciliary  body,  which  permits  it  to  be  swerved  in  different  direc- 
tions, and  the  fact  that  its  tensions  upon  the  lens  are  transmitted  to  it 
by  the  means  of  a  non-muscular  suspensory  ligament  that  is  passive 
to  the  application  of  different  muscular  tensions  or  directions  of 
tension.  A  better  basis  for  the  belief  is,  however,  direct  observa- 
tion of  its  functioning  in  children  between  the  ages  of  eight  and 
twelve,  whose  actual  myopia  becomes  apparent  or  manifest  during 
that  period  although  showing  no  previous  indications  of  it.  Beyond 
the  age  of  twelve  years  the  negative  action  merely  neutralizes  posi- 
tive accommodation.  But  further  than  this,  this  negative  function- 
ing of  the  accommodation  is  associated  with  abduction,  the  same  as 
positive  accommodation  is  associated  with  adduction,  but  in  a  less 
intimate  degree.  Optically  this  association  is  confirmed  by  slightly 
"fogging"  distant  vision  and  observing  its  effects,  especially  in  young 
people,  in  increasing  a  static  exophoria  or  reducing  a  static  eso- 
phoria,  showing  that  abduction  has  been  stimulated  by  the  accommo- 
dative effort  to  neutralize  the  effects  of  the  fogging  lens. 

Instead,  therefore,  of  confining  our  attention  strictly  to  the  very 
threadbare  subject  of  "Accommodation  and  Convergence",  we  may 
expand  it  to  include  both  sides  of  the  functional  associations,  or  make 
it  a  full  statement  of  the  associations,  as  follows : 

1.  Adduction  with  -f  Accommodation,  and 

2.  Abduction  with  — Accommodation. 

These  relationships  do  not  appear  in  normal  eyes  with  normal 
muscular  balance,  except  for  the  normal  functional  activities  due  to 
changes  in  the  distance  of  the  object ;  and  the  eyes  that  the  optome- 
trist has  most  to  do  with  are  not  usually  normal  in  either,  and  never 
in  both,  respects.     Therefore  it  is  more  important  to  consider  the 


MUSCLES   OF   THE    EYE 


133 


abnormal  states  of  the  eyes  both  with  respect  to  their  refraction  and 
muscular  balance,  in  order  to  understand  the  bearing  of  these  dif- 
ferent functionings  to  each  other,  using  the  normal  states  merely  as 
a  basis  for  comparison.  These  different  conditions  may  be  combined 
as  follows : 


Primaries: 

1  Emmetropia. 

2    Hyperopia. 

3  Myopia. 

A. 

Orthophoria    

Al 

A2 

A3 

B 

Exophoria   

Rl 

B2 

B3 

C. 

Esophoria    

CI 

C2 

C3 

We  may  refer  to  these  different  combinations  by  letter  and  num- 
ber, as  to  Ai,  B2,  C^,  etc. 


Functional  Neutrality 

The  reciprocal  inductive  influences  in  Class  Ai  offset  each 
other,  for  both  are  equally  passive  for  distant  vision  and  equally  en- 
gaged for  near  vision,  and  in  the  direction  that  satisfies  the  other. 
Hence,  they  work  in  complete  harmony.  The  objective  cau.se  for 
3  D.  of  accommodation  by  each  of  the  eyes,  is  also  the  cause  for 
3  m-a  of  convergence,  and  this  is  met  by  that  amount  of  adduction. 
Should  either  function  be  ineffective,  as  the  accommodative  function 
in  presbyopia,  this  does  not  lessen  its  influence  upon  adduction,  for 
their  relation  is  nervous  rather  than  muscular.  While  the  eyes  re- 
tain their  version  powers  unimpaired  by  age,  their  duction  power, 
especially  adduction,  gradually  subsides,  the  same  as  accommodation. 
That  is  one  reason  for  the  decentration  of  presbyopic  lenses,  or  the 
reading  segments  of  bifocals,  inward.  This  setting  of  the  segments 
in  bifocals  also  gives  direct  near  vision  through  the  centers  of  the 
segments,  and  preserves  the  neutrality  of  the  functions  in  reciprocal 
inductive  influences. 

The  optometrist  will  have  few  patients  of  this  class  visit  him  in 
a  year,  and  he  will  be  able  to  do  nothing  important  for  them  with 
lenses  for  distance.  They  will  come  in,  if  for  any  purpose,  because 
of  defects  that  his  lenses  will  not  correct,  such  as  amblyopia,  foreign 
bodies  to  be  removed  from  the  eyes,  perhaps  temporary  paralysis 
of  an  ocular  muscle,  or  opacity  of  one  of  the  dioptric  media.  These 
are  not  true  optometric  cases,  but  the  optometrist  often  possesses 


134  MUSCLES   OF   THE    EYE 

better  facilities  for  diagnosing  them  than  a  physician.  Amblyopia 
is  often  due  to  temporary  causes  that  a  physician  can  deal  with  better 
than  an  optometrist.  To  remove  foreign  bodies  the  optometrist  should 
have  an  anesthetic  for  the  purpose,  and  this  he  may  get  from  a  phy- 
sician in  his  vicinity.  Deep  seated  opacities  he  cannot  relieve,  nor 
has  the  physician  any  means,  except  surgery,  of  dealing  with  them. 
Local  paralysis,  if  temporary,  may  be  treated  by  an  optometrist  better 
than  by  a  physician,  for  the  optometrist  has  better  facilities.  The 
best  treatment  is  to  encourage  the  muscle  to  function,  which  is  done 
by  lenses  and  muscular  exercises,  both  for  ciliary  and  extrinsic 
muscles. 

Classes  B2  and  C3  may  be  functionally  neutral,  as  when  the 
static  defects  are  equal  to  each  other.  That  is,  when,  in  B2,  the 
hyperopia  and  exophoria  balance  each  other ;  or  in  C^,  when  the 
myopia  and  esophoria  equalize  each  other.  But  these  balances  are 
rarely  found.  Usually  one  of  them  predominates,  and  then  the 
balancing  of  them  optically  is  often  the  important  question  in  pro- 
viding a  correction  that  can  be  worn  with  comfort  and  satisfaction. 
It  would  be  useless  to  discuss  them  in  detail,  for  their  variations  are 
infinite.  It  is  only  important  to  note  the  effects  of  the  dominating 
influence  when  they  do  not  neutralize  each  other.  In  the  following 
the  dominating  defect  is  placed  in  the  first  column,  with  its  direct 
effects  and  inductive  influence  in  the  second  and  third  columns : 


Dominant. 

Direct. 

Inductive. 

1.     Hyperopia    

-|- Accommodation 

Adduction 

2.     Myopia    

— Accommodation 

Abduction 

Adduction 

+ Accommodation 

The  direct  effect  of  hyperopia,  as  shown  above,  is  to  accentuate 
positive  accommodation,  or  innervation  of  the  positive  ciliary  fibers, 
either  for  distant  or  near  vision.  The  influence  of  this  action  is  to 
inductively  stimulate  the  muscles  of  adduction,  primarily  the  internal 
recti,  tending  to  neutralize  or  conceal  a  real  exophoria,  or  to  induce 
apparent  orthophoria,  or  even  esophoria.  If  the  hyperopic  eyes  are 
really  orthophoric,  this  influence  may  convert  them  into  esotropia  or 
convergent  strabismus,  and  often  does  so  with  children  in  whom  the 


MUSCLES   OF   THE    EYE  135 

fusion  sense  is  not  developed.  Hence,  the  correction  of  the  hyper- 
opia neutrahzes  the  abnormal  tendency  or  turning  of  the  eyes  inward. 
In  the  second  class  above,  in  which  myopia  is  the  dominant  de- 
fect, there  may  be  combined  with  it,  orthophoria,  as  in  A3 ;  exo- 
phoria,  as  in  B3,  or  real  esophoria  as  in  C3.  As  to  the  action  of  the 
negative  ciliary  fibers,  they  are  only  effective  in  neutralizing  the 
action  of  the  positive  ones,  so  as  to  produce  the  minimum  or  static 
convexity  of  the  lens ;  but  their  inductive  effect  upon  the  muscles 
does  not  depend  upon  their  effectiveness  in  reducing  the  convexity 
of  the  lens.  Their  stimulation  for  that  purpose,  though  ineffective, 
inductively  excites  or  incites  abduction,  tending  to  give  the  eyes  an 
outward  tendency,  or  exophoria.  A  pair  of  eyes  that  are  2  D,  my- 
opic have  an  accommodative  far  point  of  3^  meter,  or  20  inches.  If 
the  eyes  are  orthophoric,  they  must  be  converged  to  this  far  point 
for  binocular  fixation.  To  do  so  there  is  required  sufficient  adduc- 
tion for  that  purpose,  as  well  as  an  additional  amount  to  neutralize 
the  influence  of  the  — accommodation,  exercised  to  hold  the  lens  to 
its  flattest  form,  as  required  for  far-point  vision,  or  to  neutralize 
the  influence  of  the  required  adduction  in  inciting  positive  accommo- 
dation. That  is,  although  a  pair  of  myopic  eyes  require  no  accommo- 
dative action  whatever  for  their  far  point,  they  do  require  to  be  con- 
verged to  it ;  and  the  adduction  that  is  necessary  for  such  conver- 
gence stimulates  positive  accommodation,  which  necessitates  an  equal 
negative  action.  To  balance  these  contending  factors,  functions  that 
would  be  normally  inactive  are  forced  into  activity. 

In  the  third  combination,  in  which  exophoria  is  the  dominant 
factor,  its  direct  effect  is  to  cause  adduction  for  distant  vision,  in 
order  to  fuse  the  images.  There  may  be  combined  with  the  exo- 
phoria a  less  degree  of  hyperopia,  B2;  emmetropia,  Bi ;  or  myopia, 
B3.  As  the  inductive  influence  of  the  adduction  is  to  cause  positive 
accommodation,  this  covers  the  hyperopia,  but  might  induce  slight 
"physiological"  myopia,  so  as  to  slightly  blur  distant  vision.  It 
would  be  more  apt  to  do  this  if  the  dominant  exophoria  were  com- 
bined with  emmetropia.  The  effect  would  therefore  be  to  produce 
simulated  or  pseudo  myopia,  which  is  but  another  name  for  "physio- 
logical" myopia.  With  the  exophoria  uncorrected,  a  pair  of  minus 
lenses   that  corrected  the  pseudo-myopia    would    improve    distant 


136  MUSCLES   OF   THE    EYE 

vision.  A  Chicago  oculist  and  teacher  expresses  his  favor  of  this 
plan  of  correction,  rather  than  to  prescribe  prisms  for  the  exophoria, 
as  the  minus  lenses  tend  to  harmonize  the  functions  of  convergence 
and  accommodation.  But  as  he  also  holds  the  idea  that  the  prism 
correction  of  the  exophoria  does  not  influence  the  accommodation,  it 
is  a  little  hard  to  reconcile  the  two  ideas.  Either  these  influences  are 
reciprocal  or  neither  has  any  inductive  influence  over  the  other.  If 
the  myopia  were  real,  it  would  tend  to  enhance  the  exophoria,  or  to 
cause  pseudo-exophoria,  the  same  as  real  exophoria  tends  to  cause 
pseudo-myopia.  If  two  neutralizing  wrongs  are  better  than  one  right, 
the  theory  of  the  oculist  referred  to  might  be  approved.  But  never- 
theless, in  cases  of  this  kind,  a  weak  prism,  base  in,  to  relax  the  ad- 
duction, invariably  improves  distant  vision,  showing  that  its  induc- 
tive influence  upon  the  ciliary  muscles  is  thereby  abated.  It  is  in 
cases  of  this  kind  that  minus  lenses  are  often  prescribed,  even  for 
slight  hyperopia,  as  they  improve  distant  vision.  A  cycloplegic  re- 
laxes the  accommodation,  but  a  weak  prism,  base  in,  does  it  equally 
well,  so  why  the  cycloplegic?  In  cases  of  apparent  weak  myopia,  it 
is  always  advisable  to  test  the  effect  on  distant  vision  of  a  weak 
prism,  base  in. 

In  the  fourth  combination,  the  real  esophoria  necessitates  the 
exercise  of  abduction  for  distant  vision,  in  order  to  maintain  fusion 
of  the  images,  but  abates  such  abduction  for  near  vision.  If  the  eso- 
phoria is  combined  with  weaker  hyperopia,  its  inductive  influence, 
since  it  is  in  the  direction  of  stimulating  — accommodation,  tends  to 
add  a  fictitious  element  to  the  real  hyperopia.  Even  if  the  eyes  are 
emmetropic,  a  pair  of  plus  spheres,  which  neutralize  the  — accom- 
modation or  tendencies,  are  worn  with  comfort,  and  without  impair- 
ment of  distant  vision.  We  usually  take  such  a  case  to  be  one  of 
weak  hyperopia,  and  regard  the  esophoria  as  simulated.  It  requires 
a  fine  drawing  of  the  lines  to  differentiate  the  two ;  but  it  will  be 
found,  eventually,  that  the  correction  of  the  esophoria,  of  a  weak 
amount  but  still  real,  will  be  worn  with  greater  comfort  and  satis- 
faction for  cases  of  Ci.  But  in  cases  of  C2,  the  plus  correction  of  the 
hyperopia  should  also  be  made.  In  cases  of  C3,  a  full  correction  of 
the  myopia  without  giving  attention  to  the  esophoria  is  not  found 
comfortable,  but  an  undercorrection  of  it,  which  amounts  to  the  same 


MUSCLES   OF   THE    EYE  137 

as  prescribing  plus  for  emmetropia,  when  combined  with  esophoria, 
will  be  worn  satisfactorily.  As  optometrists  are  so  persistently 
warned  against  prescribing  minus  lenses,  it  is  the  course  most  regu- 
larly pursued. 

Vertical  Influences 

A  vertical  imbalance  of  the  muscles,  hyperpo-  or  hypoper- 
phoria,  tends  to  give  the  eyes  an  appearance  of  esophoria.  That  is, 
either  of  the  vertical  imbalances  produces  a  tension  on  the  four  ver- 
tical recti  muscles,  two  of  them  to  neutralize  the  imbalance  and  the 
other  acting  as  checks,  so  that  all  four  of  them  exercise  a  traction 
upon  the  eyes.  As  all  four  of  these  muscles  are  anchored  to  the 
cartilaginous  ring  that  surrounds  the  optic  foramen,  which  are  nearer 
to  each  other  than  the  eyes,  or  to  the  nasal  sides  of  the  orbits,  their 
traction  upon  the  two  eyes  tends  to  converge  them.  In  the  adduc- 
tion of  the  eyes  they  co-ordinate  with  the  internal  recti,  which  is  one 
of  the  reasons  for  its  amplitude.  Hence,  it  is  advisable  to  eliminate 
any  vertical  imbalance,  or  to  neutralize  it,  before  testing  the  hori- 
zontal. A  vertical  imbalance  may  also  be  influenced,  to  a  less  degree, 
by  a  horizontal  one,  and  the  oblique  muscles  are  also  mixed  up  in 
both  vertical  and  horizontal  imbalances.  Hence,  to  determine  the 
true  muscular  condition  of  a  pair  of  eyes,  especially  for  oblique  di- 
rections of  vision,  not  only  must  refractive  or  accommodative  in- 
fluences be  neutralized,  but  the  influence  of  different  duction  pairs  of 
the  extrinsic  muscles  upon  each  other.  A  version  of  the  eyes  up- 
ward, downward,  to  right  and  to  left,  will  disclose  the  effects  of  these 
influences.  The  versions  should  be  made  after  the  muscles  have 
been  balanced  for  distance,  by  having  the  patient  turn  the  head  and 
face  in  the  opposite  direction. 

Near  Vision  Balance 

When  a  pair  of  orthophoric  eyes  are  fixing  a  small  target  at  the 
reading  distance,  or  at  any  near  distance,  adduction  is  necessary ;  and 
if  the  target  is  above  or  below  the  horizontal  medial  plane,  a  hyper- 
or  hypo-version  of  the  eyes  is  also  necessary  to  direct  vision  to  it. 
Both  the  adduction  and  the  version  are  normal  for  the  position  of  the 
target.     Primarily  that  is  the  normal  functioning  of  the  muscles,  or 


138  MUSCLES   OF  THE   EYE 

what  they  are  intended  to  do.  But  if  a  pair  of  orthophoria  eyes  are 
converged  to  a  near  target,  naturally  they  have  a  tendency  to  turn 
back  to  parallelism,  or  to  their  primary  positions  for  binocular  single 
vision  of  a  distant  target.  This  is  not  a  heterophoria  of  any  sort, 
either  exo-  or  eso-phoria.  The  eyes  also  accommodate  positively 
for  such  near  vision,  and  the  ciliary  muscles,  which  are  contracted 
for  the  purpose  of  adapting  the  refraction  to  the  near  object,  tend  to 
relax.  Any  muscle  that  is  contracted  for  a  normal  purpose  tends  to 
relax.  But  we  do  not  call  this  special  contraction  of  the  ciliary 
muscles  for  near  vision  as  indicating  hyperopia,  although  their  con- 
traction for  distant  vision  is  hyperopia.  It  takes  a  plus  lens  in  either 
case  to  relax  the  ciliary  muscles.  But  that  lens  for  distance  repre- 
sents hyperopia;  but  for  near  vision  it  represents,  if  required,  pres- 
byopia, not  hyperopia. 

It  seems  to  have  become  the  general  practice  to  give  the  eyes 
muscle  tests  for  near  vision,  and  to  call  whatever  is  found  by  the 
names, 

1.  Orthophoria- for-Near  Vision  or  Fixation, 

2.  Exophoria-for-Near  Vision  or  Fixation, 

3.  Esophoria-for-Near  Vision  or  Fixation,  etc. 

These  terms  are  analogous  to  similar  ones  expressing  the  refrac- 
tion for  near,  as  Emmetropia-for-Near,  Hyperopia-for-Near,  and 
Myopia-for-Near,  a  nomenclature  we  would  at  once  repudiate.  The 
amount  of  each  would  naturally  depend  upon  "how  near"  the  object 
or  target  is,  so  that  the  accepted  standard  reading  distance  of  ys 
meter,  or  13  inches,  is  taken  to  represent  it.  Any  pair  of  eyes  that 
accepts  a  pair  of  +2  sphs.  for  distance  with  normal  vision  unim- 
paired by  them,  will  naturally  accept  plus  lenses  of  a  higher  value 
for  13  inch  vision.  Either  eye  alone  will  accept  +3  ^-  i^  addition, 
although  both  eyes  together  may  not,  because  of  muscular  influences. 
Are  the  eyes  therefore  2  D.  hyperopia  for  distance  and  5  D.  hyper- 
opia for  near  vision?  Or  have  they  greater  hyperopia  in  a  near 
than  in  a  distant  test?  If  so,  the  term  "hyperopia"  loses  much  of 
its  significance.  It  is  equally  true  of  "exophoria-for-near"  or  eso- 
phoria-for-near."  But  as  these  expressions  are  pretty  generally  used, 
it  is  important  to  know  just  what  the  users  of  the  terms  mean  by 


MUSCLES   OF   THE   EYE  139 

them.     Therefore,  in  the  discussion  of  "Muscle  Testing"  they  will 
be  explained. 

The  Cyclophorias. 

The  cyclophorias  are  usually  the  product  of  symmetrical  oblique 
astigmatism,  or  astigmatism  in  which  the  meridians  of  maximum 
refraction,  and  also  of  minimum,  are  either  equally  divergent  up- 
ward or  downward,  as  the  vertical  and  horizontal  positions  of  these 
principal  meridians  will  not  give  the  object  an  oblique  appearance. 
Examples  of  such  astigmatism  are  those  in  which  the  maximum 
meridians,  or  meridians  of  greatest  refraction,  may  be,  for  the  right 
and  left  eye,  as  follows : 

Right,  105° ;  Left,  75° 

Right,  120°;  Left,  60° 

Right,  135° ;  Left,  45° 

Right,  150°;  Left,  30° 

Right,  165°;  Left,  15°  or  vice  versa. 

The  axes  of  the  correcting  cylinders  are  in  one  or  the  other  of 
these  principal  meridians,  according  to  whether  the  cylinder  is  plus 
or  minus,  and  the  sum  of  the  two  axial  positions  is  always  180°.  The 
astigmatism  that  the  cylinders  correct  may  be  considered  to  be  pro- 
duced artificially  by  an  opposite  cylinder  in  the  same  axis.  As  the 
eyes  cannot  accommodate  cylindrically,  but  only  for  one  meridian 
of  the  astigmatism,  it  has  no  inductive  influence  upon  the  muscles. 
The  action  is  direct,  if  it  is  anything,  and  therefore  inductively  it  is 
the  same  as  simple  hyperopia  or  myopia  in  its  inductive  influences. 


140  MUSCLES   OF   THE    EYE 

CHAPTER  IX 

Static  Photometry 

(Muscle  Testing) 

The  determination  of  the  binocular  poise  or  muscular  balance 
of  a  pair  of  eyes  requires  that  all  influences,  except  the  normal  ones 
that  pertain  to  them,  be  removed  or  eliminated.  Therefore,  for  the 
purpose  it  is  best  to  employ  a  target  that,  at  the  standard  distance  of 
6  meters  or  20  feet,  is  also  at  the  intersection  of  the  vertical  and  hori- 
zontal medial  planes.  This  distance  is  regarded  as  infinity,  and  as  it 
is  directly  in  front  of  the  eyes,  the  muscular  action  required  to  fix 
it  is  minimized  or  reduced  to  o.  But  there  is  no  outward  or  objec- 
tive indication  whether  the  muscles  are  at  rest  or  not.  That  has  to 
be  determined  by  test.  To  make  such  tests  we  require  a  suitable 
target,  and  optical  devices  of  various  kinds. 

The  Target 

The  kind  of  target  we  employ  depends  upon  what  optical  device 
we  intend  to  use  with  it.  It  must  be  rather  small,  but  large  enough 
to  be  clearly  seen  at  the  distance,  and  be  located  in  a  field  with  which 
it  is  in  clear  contrast,  such  as  white  on  black,  black  on  white,  or  a 
luminous  spot  in  a  dark  field.  A  single  line,  or  two  lines  at  right 
angles,  since  they  are  narrow  in  one  direction,  though  extended  in 
the  other,  make  a  suitable  target  for  some  purposes.  When  the  tar- 
get is  to  have  position  nearer  to  the  eyes  than  the  standard  20  feet, 
it  should  be  correspondingly  smaller,  or  may  be  printed  letters  on  a 
card,  arranged  in  a  vertical  or  horizontal  row.  But  a  near  test  of 
the  muscular  balance  involves  the  normal  use  of  the  muscles,  and 
such  normal  use  of  them  is  of  course  to  be  regarded  and  discounted 
from  the  findings.  The  balance  of  the  muscles  is  best  determined  by 
a  distance  test,  in  which  such  normal  action  of  the  muscles  is  not  in- 
volved. We  may  then  determine  the  effects  of  nearness  of  the  tar- 
get upon  them. 

The  stimulus  for  the  binocular  fixation  of  the  distant  target,  by 
which  the  muscles  are  normally  engaged,  proceeds  from  the  fusion 
of  the  images.  Whatever  the  target,  its  image  appears  upon  both 
retinas.     If  the  two  images  are  of  the  same  form,  color,  extension 


MUSCLES   OF   THE    EYE  141 

and  general  size  and  appearance,  and  naturally  take  corresponding 
positions  upon  the  two  retinas,  they  will  be  fused  into  one  visual  im- 
pression or  effect.  The  demand  for  such  fusion  will  excite  the 
muscles  to  the  required  action,  if  any  action  on  their  part  is  neces- 
sary. Therefore,  to  determine  whether  such  fusion  is  due  to  normal 
relaxation  of  the  muscles,  or  to  their  abnormal  action,  the  stimulus 
for  fusion  of  the  images  must  be  eliminated.  This  is  done  in  various 
ways  and  by  different  devices  and  targets.  We  can  either  make  the 
images  dissimilar  in  appearance,  by  the  distortion  of  one  of  them, 
or  we  may  so  deflect  the  light  into  one  of  the  eyes  that  the  image 
is  greatly  displaced,  making  it  impossible  for  the  muscles  to  so  rotate 
the  eyes,  or  one  of  them,  as  to  bring  them  to  the  relative  retinal 
positions  required  for  fusion.  By  thus  breaking  up  fusion,  causing 
diplopia,  we  eliminate  the  normal  effects  of  the  fusion  sense  as  a 
control  over  the  muscles.  As  a  consequence,  the  muscles  relax  and 
the  eyes  assume  the  positions  most  comfortable  to  them. 

When  diplopia  has  been  brought  about  in  this  way,  or  one  of 
these  ways,  we  measure  the  deviations  of  the  eyes  by  the  prism 
value  that  is  required  to  restore  the  normal  relative  positions  of  the 
images,  although  not  restoring  their  normal  appearance,  or  the  ap- 
pearance of  the  one  image  that  has  been  distorted,  discolored  or  dis- 
placed for  the  purpose  of  breaking  the  control  of  fusion  over  their 
relative  positions.  Whatever  this  prism  value  may  be,  and  what- 
ever position  it  is  required  to  stand  before  one  of  the  eyes,  or  divided 
between  the  two  of  them,  it  measures  the  tendency  of  the  eyes,  or 
of  their  visual  axes,  to  deviate  from  parallelism,  both  in  the  direc- 
tion of  the  prism  power  and  for  its  value  in  prism  diopters ;  and  this 
tendency  represents  the  muscular  tension  that  is  required  to  hold  the 
eyes,  or  their  visual  axes,  in  the  parallel  positions  required  for  the 
binocular  fixation  of  the  target. 

Device  and  Target 

As  stated,  the  target  used  depends  upon  the  devices  we  intend 
to  use  with  it ;  or  the  device  we  employ  depends  upon  the  target  to 
be  employed.  The  principal  optical  devices,  some  of  which  belong 
to  the  same  class,  and  the  target  that  is  used  with  them,  are  as 
follows : 


142  MUSCLES   OF   THE    EYE 

1.  Opaque  cover  test,  to  obscure  one  eye,  any  target. 

2.  Red  disc,  to  give  target  that  color  for  one  eye,  spot  light. 

3.  Maddox  rod,  to  make  target  a  "streak"  of  light,  spot  light. 

Cone  test,  to  make  target  circle  of  light,  spot  light. 

4.  Double  prism,  to  double  vision  of  one  eye,  single  line. 

5.  Single  prism,  to  displace  image  in  one  eye,  cross  lines. 

The  most  generally  used  of  the  above  devices  are  the  3d  and  5th, 
with  a  red  disc,  usually  to  contrast  the  images,  or  make  the  patient's 
designation  of  the  relative  positions  of  w^hat  he  sees  simple  and  clear, 
as  "the  red  light  or  line  is  to  the  left  or  right,  or  above  or  below  the 
other  one".  Device  4,  the  double  prism,  is  also  favored  by  many 
practitioners,  although  it  is  less  dependable  than  Device  3,  used  in 
the  same  way.  It  is  also  used  with  a  line  target,  making  two  lines 
of  it,  which  are  contrasted  with  the  single  line  seen  by  the  other  eye. 

While  the  use  of  these  devices,  and  the  reading  that  is  obtained 
and  what  it  means  with  respect  to  muscular  balance,  are  very  simple, 
it  is  in  the  interpretation  of  the  reading  that  many  optometrists  be- 
come confused  and  are  led  to  erroneous  conclusions,  or  misled  to 
them.  As  the  refraction  of  the  eyes  is  involved  in  them,  or  the  in- 
fluence upon  them  of  the  accommodative  function  is  to  be  regarded, 
this  is  a  further  complexity,  which  becomes  a  perplexity  to  the 
optometrist.  When  muscle  tests  are  made  for  the  reading  or  other 
near  distance,  there  are  still  further  complexities  and  perplexities  to 
be  taken  into  consideration,  and  the  combined  effects  of  all  of  these 
is  such  as  to  naturally  have  a  discouraging  effect.  If  we  add  to  all 
of  these  the  "mistaken  teachings"  to  be  found  in  some  text  books, 
and  in  the  pronouncements  of  notable  lecturers  upon  the  subject,  the 
path  of  the  student  of  optometry  is  not  an  easy  one  to  the  mastery 
of  this  special  field.  To  clear  some  of  the  perplexities  that  surround 
the  subject,  let  us  discuss  one  of  the  simplest  of  muscle  tests. 

Binocular  Test 

If,  with  a  pair  of  eyes  binocularly  fixing  the  target  at  20  feet, 
a  single  prism  of  7A  is  placed,  base  down,  before  the  right  eye, 
double  vision  of  the  target  will  usually  result.  If  the  target  is  a  pair 
of  cross  lines,  each  6  cm.  in  length,  with  3  cm.  arms  extending  from 
the  center  or  crossing  point,  this  prism  should  normally  place  one 


MUSCLES   OF  THE   EYE  143 

cross  directly  above  the  other,  separating  the  horizontal  lines  7  cm., 
or  making  them  apjiear  as  parallel  horizontal  lines  at  that  distance 
apart.  This  effect  would  separate  the  ends  of  the  vertical  lines  i  cm. 
but  maintain  their  alignment.  A  slight  variation  from  this  position 
is  not  to  count,  for  it  may  be  due  to  a  slight  inclination  of  the  prism, 
or  to  the  fact  that  normal  adduction  for  the  6  meters  (lA)  is  re- 
laxed. If,  therefore,  the  two  targets  seen  appear  in  this  relative  posi- 
tion, the  prism  accounts  for  it  and  the  appearance  is  normal. 

As  two  crosses  are  seen,  one  directly  over  the  other^  vision  may 
be  directed  to  either  one  of  them,  or  shifted  from  one  to  the  other. 
When  looking  at  the  upper  of  the  two  targets  the  right  eye  only  is 
fixing  it,  for  the  left  eye  does  not  see  it.  When  looking  at  the  lower 
target  the  left  eye  only  is  fixing  it,  for  the  right  eye  does  not  see  it. 
But  you  will  not  be  visually  conscious  of  this  fact.  It  will  seem  to 
you,  when  you  are  looking  at  either,  that  both  eyes  are  seeing  it,  that 
you  are  binocularly  fixing  it.  But  if,  while  looking  at  the  upper  tar- 
get, you  cover  the  right  eye  with  an  opaque  disc,  that  target  will  dis- 
appear ;  but  it  will  not  do  so  if  you  cover  the  left  eye  in  the  same 
way ;  and  if,  as  you  are  looking  at  the  lower  target,  you  cover  the 
right  eye,  you  will  continue  to  see  the  lower  target  as  before;  but 
covering  the  left  eye  in  the  same  manner  causes  the  lower  target  to 
disappear.  When  looking  at  either  target  your  vision  of  the  other  is 
indirect,  and  therefore  less  distinct. 

The  fact  that  in  fixing  one  of  these  targets  you  seem  to  be  see- 
ing it  with  both  eyes,  is  probably  due  to  a  reflex  of  the  visual  sensa- 
tion from  the  fovea  of  one  eye  to  the  fovea  of  the  other.  That  is,  in 
looking  at  the  upper  target,  its  image  is  placed  upon  the  fovea  of 
the  right  eye  only;  but  its  sensory  effect  in  the  brain  is  reflexed  to 
the  fovea  of  the  left  eye,  so  that  visually  it  seems  as  though  each  of 
the  foveas  were  receiving  the  same  image,  as  in  normal  binocular  fix- 
ation of  it.  But  while  fixing  one  of  the  two  visible  targets,  the  target 
indirectly  seen  by  the  other  eye  is  also  reflexed  to  the  fixing  eye,  but 
to  an  area  of  the  retina  of  that  eye  that  corresponds  with  its  position 
upon  the  retina  of  the  eye  that  sees  it  indirectly.  So  that,  either  by 
direct  fixation  of  one  of  the  eyes  and  a  sensory  reflex  to  the  other 
eye ;  or  by  indirect  vision  of  the  non-fixing  eye,  and  a  sensory  reflex 
to  the  fixing  eye,  we  may  be  said  to  be  seeing  both  targets  with  each 
eye,  or  to  have  binocular  vision  of  both  of  them.    But  vision  of  either 


144 


MUSCLES    OF   THE   EYE 


target  would  be  less  emphatic  than  real  binocular  single  vision  of  the 
cross,  for  there  is  the  light  impact  upon  but  one  retina  for  vision  of 
either  of  them,  and  but  one  visual  message  from  retina  to  brain. 
^  As  the  fovea  will  accommodate  an  image  of  .02,  or  2A,  and 
'the  image  of  a  6  cm.  target  at  6  meters  fills  but  an  angular  space 
whose  tangent  is  .01,  or  lA,  the  image  of  the  given  target,  on  the 
fovea  of  the  eye  fixing  it  will  extend  but  about  half  way  across  it, 
or  leave  an  unoccupied  border  of  3  cm.  all  the  way  around  the  cross 
that  is  a  blank  on  the  fovea.  Hence,  the  cross  that  is  not  fixed  by 
either  eye  is  imaged  with  one  of  its  arms  extending  over  the  objec- 
tive foveal  area  2  cm.,  so  that  the  crossing  point  of  the  lines  of  the 
cross  is,  in  the  non-fixing  eye,  entirely  off  or  outside  of  the  foveal 
area.  This  is  the  effect  of  the  7  cm.  displacement  of  one  of  the 
images  by  the  7A  prism.  This  will  impair  the  distinctness  of  vision 
of  the  cross  that  is  not  fixed  by  either  eye,  for  it  is  seen  by  indirect 
vision  only.  Its  sensory  reflex  to  a  corresponding  area  of  the  retina 
of  the  fixing  eye  is  therefore  correspondingly  dim.  But  by  shifting 
fixation  from  one  to  the  other  of  the  targets,  this  relationship  is  re- 
versed. If  either  eye  had  less  acuity  of  vision  than  the  other,  its  fix- 
ing vision  would  be  reduced  accordingly. 

But  the  purpose  of  producing  vertical  diplopia  in  the  above 
manner  is  not  to  determine  the  normal  consequences  of  it,  but  to 
observe  the  abnormal  ones.     This  would  be  manifest  by  any  mis- 


iRiqllt 


FIGURE  30. 

Circles  represent  areas  of  foveal  vision  at  6  meters  for  right  and  left  eyes. 
Foveal  vision  (not  images). 

Black  cross-lines  represent  vision  of  cross-line  target  of  6  cm.  as  seen  at 
6  meters,  with  7  A  prism,  base  down,  before  right  eye,  when  vision  is  directed 
to  the  upper  target.     Real  vision. 

Dotted  cross-lines  represent  reflex  vision  of  target  as  seen  by  the  other  eye. 


MUSCLES   OF  THE   EYE  145 

alignment  of  the  vertical  lines  as  seen  in  the  two  targets,  for  visually 
there  are  two  of  them.  A  horizontal  separation  of  the  two  vertical 
lines,  such  that  a  prism,  base  to  the  right  or  left,  over  either  eye, 
would  be  necessary  to  put  them  in  normal  alignment,  proves  that  the 
visual  axes  are  not  in  normal  parallelism  for  distant  vision.  They 
are  either  convergent  or  divergent,  as  the  vertical  prism  does  not 
account  for  their  horizontal  separation.  Unless  there  is  some  in- 
ductive influence,  such  as  accommodation  for  distance,  either  posi- 
tively or  negatively,  to  account  for  it,  separation  of  the  vertical  lines 
horizontally  indicates  a  horizontal  imbalance  of  the  muscles.  It  is 
one  variety  of  heterophoria. 

Diagnosis  of  Imbalance 

If  the  eyes  that  are  binocularly  tested  as  above  have  been  opti- 
cally corrected  of  any  refractive  defects,  so  that  there  is  no  accom- 
modation exercised  for  distant  vision,  then  the  direction  of  deviation 
of  the  visual  axes  is  determined  from  the  direction  of  relative  dis- 
placement of  the  two  vertical  lines  of  the  targets.  If  the  vertical 
line  in  the  upper  target,  which  is  seen  by  the  right  eye  only,  appears 
to  be  at  the  right  of  the  vertical  line  in  the  lower  target,  which  is 
seen  by  the  left  eye  only,  such  displacement  or  diplopia  is  said  to  be 
homonymous.  But  if  the  vertical  line  of  the  upper  target  appears  to 
be  to  the  leftward  of  the  vertical  line  in  the  lower  target,  then  the 
displacement  or  diplopia  is  said  to  be  heteronymous.  That  is,  in 
homonymous  diplopia,  the  right  eye  sees  the  right  hand  target,  the 
left  eye  the  left  hand  one.  In  heteronymous  diplopia  these  positions 
are  reversed,  or  the  right  eye  sees  the  left  hand  target  and  the  left 
eye  the  right  hand  one.  These  distinct  and  never  to  be  confused  re- 
sults indicate  that  the  eyes  deviate  in  a  direction  opposite  to  that  of 
the  visual  appearance.  Applied  to  the  binocular  test  above  described, 
the  results  are  as  follows  : 

I.  Homonymy.  If  the  vertical  line  in  the  upper  target  appears 
to  the  rightward  of  the  vertical  line  in  the  lower  target,  the  vertical 
Hne  in  the  lower  target  will  of  course  appear  to  the  leftward  of  the 
vertical  line  in  the  upper  target. 

This  is  homonymous  displacement  or  diplopia,  for  each  eye  sees 
the  object  in  its  own  direction,  rightward  for  the  right  eye,  leftward 


146  MUSCLES   OF  THE   EYE 

for  the  left  eye,  and  naturally  the  same  distance  of  separation  be- 
tween them. 

But  the  above  direction  of  displacement  or  diplopia  indicates 
that  the  visual  axes  of  the  two  eyes  are  convergent,  or  that  they  cross 
each  other  at  some  point,  or  would  do  so  if  extended  forward  of  the 
eyes. 

They  therefore  have  the  tendency  to  converge  and  cross  each 
other  when  binocularly  fixing  a  distant  object,  but  are  restrained 
from  doing  so  by  the  exercise  of  the  function  of  abduction. 

This  function  (abduction)  is  exercised  primarily  by  the  exter- 
nal recti  muscles,  and  they  must  be  kept  under  tension  to  maintain 
parallelism  of  the  visual  axes,  as  required  for  binocular  fixation  of 
distance,  under  these  conditions. 

This  tendency  of  the  visual  axes  to  converge,  even  for  distant 
vision,  but  which  is  restrained  by  the  function  of  abduction,  is 
termed  Esophoria,  which  is  an  abnormal  tendency  of  the  eyes  to 
converge. 

2.  Hetcronymy.  If  the  vertical  line  in  the  upper  target  appears 
to  the  leftward  of  the  vertical  line  in  the  lower  target,  that  line  in  the 
lower  target  will  of  course  appear  to  the  rightward  of  the  same  in 
the  upper  target. 

This  is  heteronymous  displacement  or  diplopia,  for  each  eye 
sees  the  object  in  the  opposite  direction  from  its  own  position,  the 
right  eye  seeing  the  target  to  the  leftward,  the  left  eye  seeing  the  one 
to  the  rightward. 

But  the  above  direction  of  displacement  of  diplopia  indicates 
that  the  visual  axes  are  divergent,  or  that  their  crossing  point,  if  pro- 
jected, is  back  of  the  eyes.  The  eyes  turn  outward,  and  must  do  so 
to  produce  such  displacement. 

They  therefore  have  the  tendency  to  diverge  in  that  manner 
when  fixing  a  distant  object,  but  are  restrained  from  actual  diver- 
gence by  the  exercise  of  the  function  of  adduction,  which  holds  the 
visual  axes  parallel. 

This  function  (adduction)  is  exercised  primarily  by  the  inter- 
nal recti  muscles,  and  they  must  be  kept  under  tension  to  maintain 
parallelism  of  the  visual  axes,  as  required  for  the  binocular  fixation 
of  a  distant  object. 


MUSCLES   OF  THE   EYE  147 

This  tendency  of  the  visual  axes  to  diverge  for  distance,  but 
which  is  resrained  by  the  function  of  adduction,  is  termed  Exo- 
phoria.  It  is  an  abnormal  tendency  of  the  eyes  to  turn  outward,  or 
away  from  each  other. 

Esophoria  Measurement 

Since,  in  esophoria,  the  displacement  or  diplopia  horizontally  is 
homonymous,  the  upper  target,  seen  by  the  right  eye,  will  appear  to 
the  rightward  of  the  lower  one,  seen  by  the  left  eye.  The  two  ver- 
tical lines  may  be  put  in  alignment  by  a  prism  that  moves  the  upper 
target  a  sufficient  distance  to  the  left.  The  prism,  for  that  purpose, 
must  be  placed  before  the  right  eye,  with  its  base  to  the  right  or  out- 
ward, as  the  deviation  of  light  toward  the  base  of  the  prism  will  ap- 
pear to  move  the  object  or  target  toward  its  apex.  If  it  takes  a  3A 
prism,  base  out  before  the  right  eye,  to  align  the  vertical  lines  of 
the  two  targets,  that  is  the  measurement  of  the  displacement  or  of 
the  esophoria.  Placing  the  same  prism  (3A)  base  to  the  left,  or  out- 
ward, before  the  left  eye  moves  the  lower  target  to  the  rightward,  or 
toward  its  apex,  the  same  distance,  and  this  will  also  put  the  vertical 
lines  of  the  cross  in  alignment.  If  there  appears  to  be  some  slight 
difference  between  the  two  positions  of  the  prism,  this  is  probably 
due  to  the  "slant"  of  the  prism,  for  a  prism  has  the  least  deviating 
effect  when  refraction  is  equal  at  its  two  surfaces,  and  it  must  have 
a  fixed  slant  for  this  purpose. 

As  the  3A  prism,  base  out  before  either  eye,  corrects  the  dis- 
placement of  the  vertical  lines,  causes  them  to  align  with  each  other, 
but  one  3A  prism,  in  either  of  these  positions,  measures  the  im- 
balance or  esophoria.  It  indicates  that  in  the  binocular  fixation  of  a 
distant  object,  or  that  in  distant  vision,  the  eyes  require  to  exercise 
3 A  of  abduction.  This  puts  a  continuous  tension  and  strain  upon 
the  external  recti  muscles.  However,  as  the  eyes  are  never  re- 
quired to  turn  beyond  parallelism  of  the  visual  axes,  this  is  not  a 
heavy  muscular  burden  to  bear.  It  makes  convergence  of  the  eyes 
for  near  vision  that  much  easier,  as  a  relaxation  of  the  external  recti 
muscles  will  allow  the  eyes  to  rotate  according  to  their  tendency,  in- 
ward, with  less  than  normal  adduction  by  the  internal  recti.    Unless 


148 


MUSCLES   OF  THE   EYE 


there  are  "reflex"  effects  upon  the  accommodative  function  more  dis- 
turbing than  the  strain  upon  the  external  recti  for  distant  vision,  an 
esophoria  is  allowed  to  go  uncorrected.  It  is  an  imbalance  that  is 
frequently  fictitious,  due  to  the  inductive  influence  of  the  accommo- 
dation in  uncorrected,  perhaps  latent,  hyperopia,  and  will  disappear 
with  the  correction  of  the  hyperopia.  But  a  true  esophoria,  one  that 
is  due  to  a  malattachment  of  one  of  the  muscles  to  the  eye,  may  not 
be  disregarded ;  especially  if  it  tends  to  exact  the  blurring  of  distant 
vision  by  an  over-plus  correction. 


FIGURE  31. 

Binocular  vision  of  cross-line  target,  with  7  A  prism,  base  down,  before 
the  right  eye,  causing  vertical  diplopia  of  7  A- 

A,  Normal  alignment  of  vertical  lines  for  above,  showing  horizontal  ortho- 
phoria. B,  Homonymous  misalignment  of  vertical  lines,  showing  esophoria. 
C,  Heteronymous  misalignment  of  vertical  lines,  showing  exophoria. 


Esophoric  Induction. 

The  inductive  influence  of  a  real  and  dominant  esophoria, 
though  it  may  be  ineffective  in  that  direction,  is  hyperopicward.  That 
is,  because  of  the  functional  action  of  abduction  to  maintain  fusion 
of  the  images  for  distant  vision,  the  crystalline  lens  is  made  to  as- 
sume its  least  convexity.  This  is  not  merely  a  relaxation  of  the 
ciliary  muscles,  but  an  active  muscular  force,  negative  accommoda- 
tion, that  holds  the  lens  down  to  its  flattest  or  least  convex  form.  A 
plus  lens  for  distance  is  usually  acceptable  in  such  a  case,  although 
it  blurs  distant  vision.  The  reason  for  this  is  that  it  makes  the  eyes 
artificially  myopic,  and  myopia  and  esophoria  are  the  best  of  pals. 
It  makes  near  vision,  at  least  a  comfort ;  for  not  only  do  the  eyes 


MUSCLES   OF   THE    EYE  149 

tend  to  turn  inward  or  converge,  without  exercising  the  function  of 
adduction,  but  the  myopia,  whether  natural  or  artificial,  relieves 
them  equally  of  exercising  normal  accommodation  for  the  distance. 
If  the  eyes  are  myopic,  without  a  plus  lens  to  make  them  artificially 
so,  then  the  esophoria  and  myopia  are  happily  joined  for  near  vision ; 
but  normal  distant  vision  is  out  of  the  question. 

If  the  eyes  are  emmetropic,  requiring  no  distant  correction,  then 
a  prism  correction  of  the  esophoria  offers  the  only  means  of  relief  to 
the  strain  upon  abduction,  the  tension  of  the  external  recti,  provided 
normal  distant  vision  is  to  be  attained.  In  myopia,  with  esophoria, 
the  myopia  and  esophoria  must  both  be  corrected  for  comfortable 
distant  vision.  The  correction  of  the  myopia  alone  will  not  suffice, 
for  the  inductive  influence  of  the  esophoria  will  make  a  full,  if  any 
correction  of  it  at  all,  unacceptable.  The  reason  for  this  is  that  it 
is  a  breaking  away  of  boon  companions,  pals,  esophoria  and  myopia, 
whose  functional  comradeship  may  not  be  disturbed  with  impunity, 
although  both  handicap  or  make  impossible  normal  distant  vision.  In 
hyperopia,  with  esophoria,  the  plus  correction  of  the  hyperopia  will 
be  greedily  accepted,  for  even  an  over-plus  correction  is  acceptable. 
But  if  hyperopia  is  the  dominant  factor,  then  the  real  esophoria  is 
likely  to  have  a  fictitious,  or  pseudo-esophoria,  added  to  it.  We  are 
speaking  here  of  the  inductive  influence  of  the  esophoria,  not  that 
of  hyperopia  or  any  other  refractive  condition. 

For  near  vision,  esophoria  with  myopia  is  no  handicap,  provided 
balance  between  the  subnormal  accommodation  and  adduction  results 
for  ordinary  near  vision  work.  This  would  be  about  in  the  ratio  of 
6A  of  esophoria  to  i  D.  of  myopia,  which  places  the  far-point  of 
accommodation  and  adduction  together  at  a  distance  of  one  meter 
from  the  eyes,  or  from  the  correction  plane  of  the  eyes,  which  is  the 
plane  of  the  lenses,  or  about  from  the  crest  of  the  bridge  of  the 
spectacle  frames  in  which  the  lenses  are  mounted.  Then,  for  the 
ordinary  reading  distance  of  13  inches,  there  would  be  necessary  co- 
ordination of  the  functions,  as  2  D.  of  accommodation  would  be  re- 
quired for  each  eye,  and  12A,  or  2  meter-angles  of  convergence. 
This  exact  ratio  is  of  course  unnecessary  but  the  accommodative 
function  is  more  exacting  than  that  of  adduction,  and  an  approxi- 
mate ratio  of  that  amount  would  balance  the  two  functions,  making 


150  MUSCLES   OF  THE   EYE 

relative  adduction  (or  convergence)  and  accommodation  harmonious 
for  near  vision.  .,..<i 

The  establishment  of  harmony  between  the  functions  is  some- 
times of  greater  importance  than  the  correction  of  either  of  the  de- 
fects individually.  Those  who  forswear  the  use  of  prisms  alto- 
gether, and  harmonize  them  on  the  refractive  side  only,  will  find  it 
necessary  to  prescribe  minus  lenses  for  emmetropes  or  hyperopes  for 
distant  vision,  or  plus  lenses  for  emmetropes  or  myopes,  according  to 
the  initial  relativity  of  the  functions.  If  a  prism  correction  is  ac- 
ceptable, and  saves  one  from  the  above  incongruous  practice,  he 
should  use  them.  They  are  just  as  essential  in  some  cases  as  spheres 
or  cylinders. 

Exophoria  Measurement 

As  exophoria,  in  the  above  muscle  test,  shows  the  upper  target 
to  the  left  of  the  lower  one,  the  right  eye  sees  this  left  hand  target. 
To  bring  the  vertical  lines  of  the  two  targets  into  alignment  by  a 
prism  over  the  right  eye,  it  must  be  placed  with  its  base  to  the  left- 
ward or  apex  to  the  rightward  over  that  eye,  so  as  to  move  the  target 
in  the  direction  of  alignment.  This  position  is  base  inward,  or  toward 
the  nose.  Over  the  left  eye  the  prism  would  have  to  be  reversed, 
but  this  would  place  the  prism  base  inward  over  that  eye.  The  latter 
moves,  or  appears  to  move,  the  lower  target  to  the  leftward,  or  in  the 
direction  of  alignment  of  the  vertical  lines  of  the  two  targets  with 
each  other.  It  is  obvious  that  it  is  as  many  A's  of  displacement  one 
way  or  the  other,  or  that  the  displacement  is  relative,  and  that  it  is 
the  same  distance  from  one  to  the  other  as  it  is  from  the  other  to  one. 
The  prism  that,  base  in  before  the  right  eye,  aligns  them,  will  also 
align  them  if  placed  base  in  before  the  left  eye. 

If  a  5A  prism  is  necessary  in  one  case,  it  is  necessary  in  the 
other  also.  Although  either  prism  is  placed  base  in,  that  is  a  reverse 
position  for  the  two  eyes.  One  moves  the  upper  target  to  the  right, 
the  other  moves  the  lower  target  to  the  left.  If  we  divide  the  prism 
into  two  equal  segments  of  23^  A  each,  and  place  one  of  these  half- 
values  over  each  eye,  but  both  with  their  bases  inward,  we  will  get 
the  same  effect ;  for  the  prism  before  the  right  eye  moves  the  upper 
target  23^2 A  to  the  rightward;  while  the  prism  before  the  left  eye 


MUSCLES   OF  THE   EYE  151 

moves  the  lower  target  2}^  A  to  the  leftward,  and  this  unites  the  ver- 
tical lines  of  the  targets  the  same  as  if  one  of  them  had  been  moved 
the  entire  distance.  While  this  is  not  the  best  test  for  horizontal 
imbalances,  because  a  vertical  imbalance  may  disturb  the  relative 
positions  of  the  two  targets  vertically,  it  is  perfectly  clear  what  these 
misalignments  mean.  They  clearly  indicate  that  the  visual  axes  are 
not  parallel,  but  are,  in  this  case,  divergent,  showing  exophoria. 

Many  optometrists  are  puzzled  over  the  apparent  inconsistency 
of  the  outward  turning  of  the  eyes  causing  crossed  visual  diplopia. 
But  it  is  very  simple  and  obvious.  If  the  right  eye  sees  the  target 
to  the  leftward,  its  image  is  necessarily  upon  the  temporal  side  of  its 
retina,  or  temporalward  relative  to  that  seen  by  the  left  eye ;  if  the 
left  eye  sees  the  target  to  the  rightward,  its  image  is  necessarily  upon 
the  temporal  side  of  its  retina,  or  temporalward  relative  to  the  posi- 
tion of  the  image  on  the  right  retina ;  or  both  are  temporalward  from 
the  foveas.  The  eyes  have  a  position,  or  relative  positions  such  that 
the  two  foveas  are  inward  or  nasalward.  Hence  the  visual  axes, 
which  begin  at  the  foveas,  must  spread  or  diverge  in  front  of  the 
eyes,  as  they  begin  to  do  so  within  the  eye  balls  or  globes.  The  eyes 
are  turned  in  the  opposite  direction  from  that  in  which  the  targets 
appear.  If  the  targets  are  deviated  outward  (homonymy)  the  eyes 
deviate  inward ;  if  the  targets  are  deviated  inward  (heteronymy)  the 
eyes  deviate  outward.  Or,  in  other  words,  homonymy  of  vision  is 
heteronymy  of  deviation ;  and  heteronymy  of  vision  is  homonymy  of 
deviation. 

Whatever  exophoria  appears,  under  a  muscle  test,  is  genuine. 
It  is  even  more,  seldom  or  never  less,  than  it  appears.  It  may  be 
concealed  by  the  inductive  influence  of  dominant  hyperopia,  con- 
verted into  apparent  orthophoria,  or  even  manifest  apparent  eso- 
phoria.  But  this  is  the  inductive  influence  of  hyperopia  upon  mus- 
cular balance,  or  apparent  muscular  balance  or  imbalance.  If  the 
exophoria  is  dominant,  it  will  be  that  which  influences  the  apparent 
refraction  instead  of  the  other  way  about.  It  seems  fairly  obvious 
that,  if  the  muscular  condition  causing  adduction  or  abduction,  in- 
ductively influences  the  refraction  or  accommodation,  the  refractive 
condition  or  accommodation  \w\\\  inductively  influence  the  muscular 
condition,  or  apparent  condition.    There  is  this  difference,  however, 


152  MUSCLES   OF  THE   EYE 

positive  accommodation  has  its  high  point  or  maximum  power,  vary- 
ing with  age ;  and  its  recession  from  that  high  point  downward  takes 
it  to  zero  or  o,  and  the  latter  is  negative  accommodation.     But  in 


T4 

r  g*-! 

/ 

0 

as 

1 

O.D, 

z 

0 

11 

a  u 

B 

ii 

O.LI. 

^ 

0 

'1 

§ 

o 

FIGURE  32. 

Objective  appearances  in  Maddox  rod  test  of  horizontal  balance  of  muscles: 
1,  target  as  6  meters,  small  bright  light;  2,  as  seen  by  right  and  left  eyes, 
vertical  streak  in  right  eye;  3,  as  seen  in  orthophoria,  light  in  streak;  4,  as 
seen  in  esophoria,  homonymous  displacement;  5,  as  seen  in  exophoria,  heter- 
onymous   displacement. 

muscular  balance,  the  zero  point  is  between  the  two  opposite  func- 
tions of  adduction  and  abduction,  and  the  functions  extend  in  either 
of  two  opposite  directions  from  this  zero  point. 

Exophoric  Induction 

The  influence  of  exophoria  upon  the  refraction  or  accommoda- 
tion of  the  eyes  is  myopicward.  That  is,  since  it  excites  the  function 
of  adduction  to  obtain  and  maintain  binocular  fixation  of  a  distant 
object,  the  internal  recti  muscles  are  under  tension  for  distance,  and 
greater  than  normal  tension  for  near  vision,  the  same  as  the  ciliary 
for  hyperopia.  The  stimulas  for  both  of  these  actions  is  supplied 
through  branches  of  the  3d  pair  of  cranial  nerves,  the  motor  oculi. 
As  a  reflex  action,  the  excitation  or  innervation  of  one  of  these 


MUSCLES   OF   THE    EYE  153 

functions  naturally  excites  or  stimulates  the  other,  and  through  a 
common  ganglionic  reflex  nerve  center.  This  is  the  accepted  ex- 
planation of  their  close  association,  "Accommodation  and  Converg- 
ence" as  they  are  usually  termed.  A  better  designation  of  the  fac- 
tors of  association  are  "Positive  Accommodation  and  Adduction" 
for  adduction  is  the  function  and  convergence  is  the  effect.  In  the 
adduction  of  a  pair  of  exophoric  eyes  for  distant  vision,  which 
merely  parallels  the  visual  axes,  positive  accommodation  is  induc- 
tively excited  or  stimulated  the  same  as  by  adduction  for  near  vision. 

But  the  development  of  this  association  has  been  through  the 
necessity  for  common  or  joint  action  for  near  vision.  Nearness  of 
the  object  normally  excites  the  accommodation  to  maintain  focaliza- 
tion  of  light  at  the  retinas  of  the  two  eyes,  and  thus  make  the  retinal 
images  clear.  It  also  normally  excites  adduction  to  converge  the 
two  eyes  and  maintain  binocular  fixation,  so  as  to  maintain  fusion 
of  the  images  and  single  vision.  The  two  functions  therefore  nor- 
mally co-ordinate,  each  with  the  other.  This  has  made  it  a  universal 
habit,  and  heredity  perpetuates  the  habit  from  generation  to  genera- 
tion ;  and  these  combined  influences,  one  of  which  is  the  fact  that  the 
two  functions  are  stimulated  by  motor  nerves  having  a  common  re- 
flex ganglionic  origin  or  nerve  center  is  but  one.  In  fact  it  may 
have  been,  or  probably  was,  the  physical  necessity,  followed  by 
the  development  of  the  habit,  and  these  with  heredity  to  perpetuate 
them,  that  evolved,  in  human  anatomy  and  physiology,  the  neuro- 
muscular association  of  the  two  functions. 

But  the  fact  that,  from  some  inequality  of  attachment  of  the 
muscles  to  the  eye-balls,  adduction  for  distance  is  demanded,  does 
not  lessen  its  inductive  influence  in  exciting  the  accommodation  to 
joint  action  with  it.  As  exophoria  puts  such  a  demand  upon  adduc- 
tion, the  accommodation  is  stimulated  by  it  to  a  greater  or  less  de- 
gree. What  the  consequences  of  this  inductive  stimulation  of  the 
accommodation  are  depends  upon  the  refractive  condition  of  the 
eyes.  If  the  eyes  are  hyperopic,  and  for  an  equivalent  amount,  then 
the  two  functional  actions  will  be  happily  joined.  They  are  natural 
pals,  and  go  hand-in-hand  over  the  entire  range  of  vision,  from  dis- 
tance to  near.    There  is  an  intrinsic  demand  upon  each  function,  and 


154  MUSCLES   OF   THE   EYE 

each  inductively  co-ordinates  with  the  other.  Although  each  is  optic- 
ally abnormal,  their  abnormalities  fit  each  other,  and  they  become 
boon  companions  in  exercising  the  muscular  functions  involved  in 
vision.  But,  they  will  not  pleasantly  accept  separation.  Like  Jack 
and  Jill,  or  John-Anderson-my-Jo-John,  they  climb  life's  hill  to- 
gether. So,  if  Jack  comes  tumbling  down,  and  even  though  he  may 
break  his  crown,  Jill  comes  tumbling  after. 

Functional  Equilibrium 

The  two  functions  are  in  accord,  unison  or  equilibrium  when, 
for  a  width  of  6  cm.  between  the  eyes,  there  is  6A  of  adduction 
necessary  by  the  eyes  jointly,  for  i  D.  of  accommodation  by  the 
eyes  individually.  This  puts  the  two  functions  in  the  ratio  of  6A  to 
I  D.,  or  6  to  I.  Measured  in  meter  angles,  there  is  i  m-a  of 
adduction  to  i  D.  of  accommodation  by  each  eye.  Although  the 
accommodation  is  exercised  by  the  eyes  individually,  their  associated 
action  is  about  as  intimate  as  the  joint  action  of  adduction.  When, 
therefore,  there  is  6A  of  exophoria  to  i  D.  of  hyperopia  in  each  eye, 
the  two  functions  are  in  normal  equilibrium.  As  adduction  and 
accommodation  increase  normally  in  the  same  ratio  for  near  vision, 
this  equilibrium  is  maintained  for  all  distances  of  the  object,  or 
relative  adduction  and  accommodation  is  normal,  although  neither 
may  be  individually  normal.  The  two  functions  are  then,  not  only 
pals  for  distant  vision,  but  for  near  vision  also.  We  can  get  along 
very  well  however  when  this  relationship  is  quite  far  from  normal, 
as  the  functions  adapt  themselves  to  a  considerable  difference.  Like 
a  person  with  a  short  leg,  a  slight  hip  adjustment  equalizes  the  two. 

When  the  hyperopia  exceeds  this  relative  value,  it  becomes  the 
dominant  factor  in  the  relationship.  But  if,  with  exophoria  of  a 
considerable  amount,  there  is  weaker  hyperopia,  emmetropia  or 
myopia,  the  exophoria  is  dominant.  It  tends  to  nullify  or  conceal 
the  hyperopia,  makes  the  eyes  appear  emmetropic  or  slightly  myopic, 
so  that  the  plus  lenses  that  correct  the  eyes  individually  are  not  com- 
fortable or  acceptable  binocularly ;  the  emmetropic  eyes  are  made  to 
see  distant  objects  better  with  weak  minus  lenses ;  and  the  myope 
demands  more  minus  than  the  eyes  individually  call  for.    There  are 


MUSCLES   OF   THE   EYE  155 

some  practitioners,  medical  as  well  as  optometrical,  who  have  such 
an  antipathy  to  prisms  that  they  prefer  to  prescribe  minus  lenses  in 
these  cases ;  and  some  who  don't  know  any  better,  which  also  in- 
cludes oculists  as  well  as  optometrists.  The  oculist  who  uses  drops 
is  apt  to  find  out  that  less  minus  or  none  is  called  for.  The  optome- 
trist could  find  it  out  in  a  much  simpler  way,  by  trying  the  effects 
on  distant  binocular  vision  of  a  weak  prism  (2 A  to  4A)  base  in, 
held  before  either  eye.  Improvement  will  then  show  that,  by  relax- 
ing adduction,  the  ciliary  tension  is  also  relaxed,  proving  the  myopia 
to  be  fictitious  to  that  extent.  It  is  physiological  myopia,  which  is 
but  another  name  for  false-,  simulated-  or  psuedo-myopia. 

In  the  same  way  that  exophoria  affects  the  apparent  refraction 
of  the  eyes,  hyperopia  afifects  their  apparent  muscular  balance,  when 
it  is  the  dominant  factor.  The  accommodation  required  for  distant 
vision  excites  the  positive  ciliary  fibers,  and  their  innervation  and  the 
innervation  of  the  internal  recti  are  simultaneous  or  synchronous. 
Hence,  a  muscle  test  for  distance  with  the  hyperopia  uncorrected, 
does  not  relieve  the  ciliary  of  the  action  required  to  focus  the  target. 
But  since,  under  a  muscle  test,  there  is  no  longer  the  fusion  stimulus 
to  determine  what  muscular  action  is  necessary  to  fuse  the  images, 
the  eyes  and  the  muscles  respond  to  any  extraneous  influence  that 
may  be  brought  to  bear  on  them.  The  accommodative  action  induc- 
tively excites  adduction,  so  that,  instead  of  assuming  their  positions 
of  rest,  adduction  turns  the  visual  axes  toward  each  other.  Some 
and  perhaps  all  of  their  tendency  to  diverge  is  overcome ;  or  they 
may  even  be  converged  slightly.  The  relative  positions  of  the  tar- 
get will  then  show  but  a  part  of  their  actual  exophoria ;  or  it  may 
all  be  covered  by  the  adduction  and  show  apparent  orthophoria ; 
or  if  the  association  is  intimate,  esophoria  may  be  apparent.  It  is 
fictitious,  not  a  real  structural  imbalance  of  the  muscles  in  that  direc- 
tion. This  also  is  physiological  esophoria,  which  is  but  another  name 
for  false-,  simulated-  or  psuedo-esophoria.  But  it  is  the  most  fre- 
quent form  of  esophoria  encountered,  the  most  prevalent  form.  In 
children  it  is  apt  to  cause  such  adduction  to  manifest  itself  as  con- 
vergent strabismus,  which  is  none  the  less  manifest,  in  spite  of  the 
static  exophoria.  A  full  correction  of  the  hyperopia  will  remove  this 
influence,  provided  we  can  find  it  under  these  circumstances. 


156  MUSCLES   OF  THE   EYE 

In  the  same  way  that  excessive  near  work  may  result  in  cihary 
spasm,  not  only  in  hyperopia  and  emmetropia,  but  in  weak  myopia, 
ciliary  action  that  is  due  to  the  inductive  influence  of  exophoria  may 
foster  it.  Hyperopia  ceases  to  be  the  dominant  factor  with  the  cor- 
rection of  the  manifest  amount  with  plus  lenses;  but  with  the  exo- 
phoric  influence  remaining,  a  concealed  element  of  ciliary  action  re- 
mains, and  this  connot  be  "fogged"  out  with  repression  plus  lenses. 
But,  we  can  relax  and  repress  the  abnormal  adduction  by  prisms  so 
that  only  ciliary  spasm,  due  to  hyperopia,  is  left.  Hence,  repression 
lenses,  to  fully  relieve  all  influences  tending  to  excite  or  incite  ciliary 
action,  must  embody  both  factors :  the  plus  sphericals  to  directly 
relieve  ciliary  action,  and  the  prisms  base  in  to  relieve  and  repress 
abnormal  adduction,  due  to  exophoria.  The  prisms  will  relieve  the 
ciliary  of  the  influence  of  exophoria  much  more  readily  than  spheric- 
als, but  neither  without  the  other  is  complete. 

Vertical  Tests 

In  describing  tests  of  the  vertical  muscles  for  imbalance,  the 
Maddox  rod  will  be  employed  to  make  the  tests,  instead  of  the  single 
prism.  It  is  perhaps  a  better  test  for  the  horizontal  imbalances  also, 
and  a  figure  to  show  the  dififerent  results  of  this  test  of  the  horizontal 
meridians  and  muscles  is  shown  on  another  page.  The  target  for  a 
Maddox  rod  test  is  a  small  but  clear  light  at  6  meters,  one  that  is 
I  cm.  in  diameter,  if  distinct  and  in  a  dark  field,  will  show  the  devia- 
tions or  displacem.ents  better  than  a  larger  one.  If  the  Maddox  rod 
is  multiple  and  the  aperture  in  the  opaque  disc  is  round,  better  results 
are  obtained  than  with  the  single  rod.  If  the  Maddox  rod  is  of 
uncolored  glass,  the  other  eye  may  be  covered  by  a  red  disc,  so  as  to 
facilitate  the  patient's  description  of  its  location.  The  Maddox  rod 
converts  the  target  into  a  fine  but  bright  streak  of  light.  When  the 
rod  is  vertical,  or  has  the  rod  or  ridges  vertical  over  the  eye  before 
which  it  is  placed,  that  eye  will  see  a  horizontal  streak  of  light, 
while  the  other  eye  sees  the  bright  spot  openly  and  naturally. 

As  the  visual  sense  does  not  recognize  the  identity  of  the  two 
visual  effects,  but  visualizes  the  result  as  "a  light  and  a  streak," 
there  is  no  natural  reflex  stimulus  to  bring  two  such  dissimilar  im- 


MUSCLES   OF   THE    EYE 


157 


ages  together,  or  into  the  relative  positions  for  fusion.  Hence,  if 
they  assume  that  position,  with  the  streak  running  through  the  Hght, 
or  the  red  light  is  in  the  streak,  it  is  because  there  is  no  tendency  of 
the  eyes  to  deviate  from  normal  parallelism  for  distant  vision.  But 
if  the  red  light  takes  a  position  above  or  below  the  streak,  that  indi- 
cates that  the  visual  axes  are  not  parallel,  but  that  one  is  directed 
higher  than  the  other.  If  it  should  be  the  right  eye  that  is  directed 
higher  than  the  left,  the  left  eye  would  of  course  be  directed  lower 
than  the  right.  It  is  a  relative  matter  altogether.  Binocular  atten- 
tion or  vision  can  be  fixed  upon  either,  but  of  course  only  one  of 
the  eyes  sees  either  streak  or  light  by  direct  vision.  We  may,  how- 
ever, by  the  use  of  a  prism,  base  up  before  one  of  the  eyes,  or  base 
down  before  the  other,  bring  the  two  together.  They  do  not  fuse, 
but  merely  cross  corresponding  subjective  points  of  fixation  in  the 
two  eyes,  or  rest  upon  them. 


T4^ 

9».t 

0 

O.S. 

o 

D. 

o 

■    ss.z 

--■ 

---" 

---- 

O.U. 

-zOr:i- 

■-> 

o.  u. 

o 

O.U. 

o 

FIGURE   33. 


Objective  appearance  in  Maddox  rod  test  of  vertical  balance  of  muscles: 
1,  target  at  6  meters,  small  bright  light;  2,  as  seen  by  right  and  left  eye, 
horizontal  streak  in  right  eye;  3,  appearance  in  muscular  balance  or  ortho- 
phoria; 4,  as  seen  in  hyperpo-phoria,  right  eye  higher  than  left;  5,  as  seen 
in  hypoperphorja,   left  eye   higher  than  right. 


158  MUSCLES   OF  THE   EYE 

If  the  Maddox  rod  is  before  the  right  eye  and  the  red  disc  is  be- 
fore the  left  eye,  and  the  red  light  is  in  the  streak  centrally,  then 
the  visual  axes  are  parallel,  and  there  is  no  tendency  of  the  visual 
axes  to  deviate  relative  to  each  other  vertically.  This  indicates  ver- 
tical orthophoria  or  binocular  balance  of  the  eyes  or  muscles,  for 
there  is  no  fusion  stimulus  to  cause  them  to  assume  that  position. 
If  the  eyes  are  hyperopic,  accommodation,  by  inciting  adduction,  may 
put  a  slight  tension  on  the  four  vertical  muscles,  but  it  will  be  equal 
on  all  of  them,  if  the  object  is  straight  before  the  eyes,  and  this  will 
cause  no  vertical  deviation  of  the  visual  axes  relative  to  each  other. 
If  the  object  or  target  is  near,  this  also  will  cause  a  slight  tension  on 
the  verticals,  but  it  will  be  equal.  Version  of  the  eyes  in  any  direc- 
tion may  disturb  the  equiUbrium,  but  only  to  a  slight  extent,  if  the 
muscles  are  normally  innervated.  Paralysis  or  paresis  of  a  vertical 
muscle  would  make  normal  version  in  the  direction  of  the  affected 
muscle  impossible,  and  this  immobility  would  cause  streak  and  light 
to  separate. 

If  the  red  light  is  not  in  the  streak,  but  above  it,  the  left  eye  is 
directed  relatively  lower  than  the  right  eye,  or  its  fovea  is  raised 
above  a  corresponding  position  with  that  of  the  right  eye,  so  that  the 
image  occupies  a  relatively  lower  position  in  the  left  eye  than  in  the 
right  eye,  and  is  projected  to  a  position  relatively  higher  than  the 
image  or  streak  in  the  right  eye.  As  the  fusion  sense  is  not  in  con- 
trol of  their  positions,  the  left  eye  tends  to  turn  lower  than  the  right 
and  does  so  turn  in  the  test.  This  is  but  to  say  that  the  right  eye 
has  a  tendency  to  turn  higher  than  the  left,  and  does  so  turn  in  the 
test.  This  imbalance  of  the  muscles  has  been  termed  right  hyper- 
phoria, which  is  an  indictment  of  the  right  eye  as  the  eye  at  fault. 
But,  as  the  deviation  and  the  tendency  are  merely  relative,  it  de- 
serves a  more  significant  designation.  We  use  the  term  Hyperpo- 
phoria  to  designate  it,  and  Hypoper-phoria  to  designate  the  opposite 
imbalance.  This  avoids  the  unwieldy  compounds  "hyper-hypo"  and 
"hypo-hyper"  as  prefixes  to  the  general  term  'phoria.  The  com- 
mon designation  for  the  latter  imbalance  is  left  hyperphoria,  which 
attributes  the  imbalance  to  the  left  eye  and  discharges  the  right  eye 
from  any  responsibility. 


MUSCLES   OF   THE    EYE  159 

Muscular  Neutralization 

To  neutralize  a  tendency  of  the  right  eye  to  turn  higher  than 
the  left  eye,  and  to  fuse  the  images  in  spite  of  such  tendency,  there 
must  be  a  contraction  of  the  inferior  rectus  of  the  right  eye  and 
the  superior  rectus  of  the  left  eye.  Naturally  the  tension  of  these 
two  muscles  to  hold  the  visual  axes  on  the  same  level  is  equal.  It  is 
a  "duction,"  for  it  is  ''An  involuntary  muscular  action  that  normally 
rotates  the  two  eyes  equally  in  opposite  directions."  In  this  case  it 
merely  restrains  the  eyes  from  abnormally  rotating  in  that  manner, 
as  a  pair  of  muscles  always  does  in  any  'phoria.  The  duction  re- 
quired for  hyperpo-phoria  may  be  termed  sumsur-duction  or  hypoper- 
duction,  for  the  duction  required  to  overcome  a  'phoria,  in  any  direc- 
tion, is  in  an  opposite  direction  from  the  tendency.  Hence,  hyperpo- 
phoria  puts  a  muscular  tension  on  the  inferior  rectus  of  the  right 
eye  and  superior  rectus  of  the  left  eye.  The  opposite  tendency, 
hypoper-phoria,  puts  muscular  tension  upon  the  superior  rectus  of 
the  right  eye  and  inferior  rectus  of  the  left  eye,  and  is  sursum- 
duction,  or  hyperpo-duction,  as  you  choose. 

An  imbalance  of  the  eyes  vertically,  or  of  the  vertical  muscles, 
as  all  four  of  them  are  involved  in  one  way  or  another,  puts  an  undue 
tension  and  strain  upon  one  or  the  other  of  the  vertical  duction  pairs 
of  muscles,  the  opposite  pair  being  passive  or  non-resisting,  except 
to  restrain  the  tension  pair  from  over-action.  This  tension  must  be 
kept  up  to  prevent  the  eyes  from  deviating,  which  would  at  once  re- 
sult in  vertical  diplopia,  seeing  two  objects  for  every  one,  one  above 
the  other.  As  the  vertical  duction  powers,  sursum-duction  and 
sumsur-duction,  are  quite  limited,  not  from  "weakness"  of  the  mus- 
cles, but  inability  to  innervate  them  in  the  peculiar  manner  required 
(the  superior  rectus  of  one  eye  with  the  inferior  rectus  of  the  other) 
a  slight  imbalance,  measured  by  a  lA  or  a  2 A  prism,  may  cause 
more  discomfort  than  a  much  greater  horizontal  imbalance.  There 
are  rare  cases  in  which  such  an  imbalance  reaches  3,  4,  5  or  more 
A's,  that  the  muscles  overcome ;  but  usually  in  such  an  inequality  of 
tension,  one  of  the  eyes  turns  in  the  direction  of  its  tendency,  pro- 
ducing a  vertical  strabismus,  and  causing  repression  of  vision  in 


160  MUSCLES   OF   THE   EYE 

the  deviating  eye,  or  at  least  the  use  of  the  eyes  alternately  for 
visual  purposes.  The  continuous  repression  of  vision  in  one  of  the 
eyes  causes  its  acuity  of  vision  to  wane,  or  the  eye  to  become  ambly- 
opic. 

Prism  Measurement 

The  degree  of  the  vertical  imbalance  is  determined  by  the  value 
of  the  prism  required,  in  the  test,  to  move  the  light  into  the  streak, 
or  the  streak  to  the  light.  In  hyperpo-phoria  the  prism  is  placed  base 
down  before  the  right  eye  or  base  up  before  the  left ;  and  in  hypoper- 
phoria  it  is  placed  base  down  before  the  left  eye  or  base  up  before 
the  right.  The  prism  value  that  is  required,  in  either  case,  before  one 
eye,  is  the  measure  of  the  imbalance  in  prism  diopters.  No  atten- 
tion need  be  paid  to  any  slight  difference  in  their  values,  if  the  meas- 
urement is  made  in  both  directions,  that  is,  first  base  down  before  one 
eye  and  then  base  up  before  the  other,  for  such  differences,  if  any, 
are  due  to  the  different  slants  of  the  prism  in  the  mounting,  on  one 
side  and  then  the  other.  If  a  2A  prism,  base  down  before  the  right 
eye,  brings  the  streak  up  to  the  light,  a  2A  prism,  base  up  before  the 
left  eye,  will  bring  the  light  down  into  the  streak.  The  imbalance  is 
then  2A  of  hyperpo-phoria  (or  right  hyper-phoria  as  it  is  generally 
called).  Turning  both  eyes  upward  or  downward  back  of  the  prism 
and  red  disc  should  cause  but  slight  change.  To  turn  the  eyes  in  this 
manner  with  a  fixed  target,  the  head  must  be  tilted  in  the  opposite 
direction  to  maintain  fixation. 

Much  greater  caution  is  required  for  prescribing  a  prism  or 
pair  of  them  for  a  horizontal  imbalance  than  for  a  vertical  one. 
There  are  scarcely  any  inductive  influences  to  falsify  or  masquerade 
a  vertical  imbalance.  There  is  much  greater  certainty  of  giving  the 
required  relief  to  the  muscles  in  prescribing  a  prism  correction  of 
vertical  imbalances  than  for  a  horizantal  one.  The  question  of  the 
apportionment  of  the  value  of  the  prism  between  the  two  eyes  is  one 
of  mechanical  convenience  or  advantage.  The  entire  value  may  be 
before  either  eye,  base  down  before  one  or  base  up  before  the  other ; 
or  its  value  may  be  equally  divided  between  the  two.     In  the  fol- 


MUSCLES   OF   THE   EYE  161 

lowing,  three  different  arrangements  for  a  4A,  correction  of  hyper- 
po-phoria  is  shown, 

1.  4 A  base  down,  before  the  right  eye,  only. 

2.  4A  base  up  before  the  left  eye,  only. 

3.  2  A  base  down  over  right,  2  A  base  up  over  left. 

The  symptoms  that  an  uncorrected  vertical  imbalance  produces 
are  asthenopia  in  any  of  its  forms,  eye  distress  or  pain,  headaches 
and  general  organic  derangement,  from  reflex  influences,  and  very 
often  an  unconquerable  sleepiness  that  makes  night  reading  or  study 
impossible.  In  attempting  to  do  night  work  or  reading,  the  reader 
soon  finds  that  his  eye-lids  become  too  heavy  to  allow  the  eyes  to 
be  kept  open,  and  he  goes  to  sleep  over  his  book.  The  effect  of  a 
prism  correction  of  his  hyperpo-  or  hypoper-phoria  is  magical  in  its 
effects  in  relieving  this  influence.  With  the  prism  correction  he  reads 
for  hours  unaware  of  the  time  as  the  hours  slip  away.  A  lA  cor- 
rection has  a  relieving  effect  apparently  out  of  all  proportion  to  its 
value.  As  lenses  of  weak  power,  spherical,  cyHndrical  or  compound, 
are  often  more  effective  in  relieving  asthenopia  than  stronger  ones, 
although  they  may  have  but  slight  effects  in  improving  vision,  so 
the  correction  of  a  weak  vertical  imbalance  is  effective  in  the  same 
way. 

A  prism  or  prisms,  prescribed  for  permanent  wearing,  are  not 
given  for  the  purpose  of  paralleling  the  visual  axes,  or  to  "straighten"' 
the  eyes ;  but  to  relieve  the  muscles  under  strain  because  of  the  im- 
balance. Instead  of  straightening  the  eyes,  the  prism  allows  the  eye 
to  deviate  according  to  their  tendency,  thus  putting  the  subjective 
points  of  fixation  at  the  foveas  out  of  exact  alignment  with  the  objec- 
tive point.  But  the  prism  so  deviates  the  course  of  light  from  the 
objective  point  that  it  falls  upon  the  subjective  foveal  points,  thus 
putting  the  images  in  correct  relative  positions  for  fusion.  The  prism 
has  no  muscles,  nor  has  it  motor  nerves  to  be  put  under  stress  and 
strain.  Its  form  and  refraction  take  these  muscular  and  nervous  ten- 
sions off  of  the  physiological  organs,  so  that  the  eyes  may  exercise 
their  functions  normally  and  without  pain  and  distress.  Improved 
vision,  though  important,  and  usually  brought  about  by  lenses,  is 


162  MUSCLES   OF  THE   EYE 

less  important  than  the  relief  of  muscles  of  abnormal  tensions,  and 
motor  nerves  of  exercising  abnormal  control. 

Oblique  Muscle  Tests 

1^0  determine  whether  the  oblique  muscles  are  in  balance 
or  not,  it  is  necessary  to  eliminate  the  operation  of  the  fusion  con- 
trol that  causes  a  horizontal  line  target  to  appear  horizontal,  and  a 
vertical  line  target  to  appear  vertical,  to  both  eyes,  so  that  they  visu- 
ally coincide.  Perhaps  the  simplest  way  to  do  this  is  by  placing  the 
double-prism  before  one  of  the  eyes,  with  the  bases  of  the  prisms 
extending  in  the  same  direction  as  the  line-target.  This  will  make 
the  single  line  of  the  target  appear  double  to  the  eye  over  which  the 
prism  is  placed.  The  double  line  will  appear  as  two  parallel  lines, 
separated  according  to  the  power  of  the  double  prisms,  and  the  dis- 
tance of  the  target.  Two  lA  prisms  in  the  double-prism,  for  a  6 
meter  distance  of  the  target,  would  separate  the  parallel  lines  .02  of 
600  cm.  =  12  cm.,  one  up  and  the  other  down,  or  one  to  the  right 
and  the  other  to  the  left,  both  being  "displaced"  equally  6  cm.  in 
opposite  directions. 

If  the  target  is  a  horizontal  line,  and  the  double  prism  is  placed 
before  the  right  eye,  with  the  line  between  the  prisms  horizontal,  that 
eye  will  see  the  target  as  two  parallel  horizontal  lines.  The  left  eye, 
uncovered,  or  covered  by  a  red  glass  disc,  sees  the  single  line,  but  not 
displaced  if  the  muscles  are  in  balance,  and  normally  it  should  be 
midway  between  the  two  parallel  lines,  and  parallel  with  them.  It  is 
the  parallelism  of  this  third  line,  seen  by  the  left  eye,  with  the  other 
two,  or  their  parallelism  with  it,  that  determines  whether  the  oblique 
muscles  are  under  equal  tension  or  equally  relaxed.  Its  nearness  to 
one  of  the  parallels,  or  less  distance  from  it  than  the  other,  indicates 
a  vertical  imbalance,  but  not  a  cyclo-phoria.  If,  however,  it  has  a 
slanting  direction,  relative  to  the  parallel  lines,  this  indicates  a  cyclo- 
phoria,  made  manifest  by  the  test.  There  is  no  prism  action  that 
would  rectify  this  efifect,  as  a  prism,  base  down  or  up,  would  merely 
raise  or  lower  what  the  eye  it  covers  sees,  two  lines  or  one.  There  is 
no  kind  of  lens,  except  a  cylinder,  to  change  the  slant  of  the  target 
as  seen  by  either  eye ;  and  without  astigmatism  to  correspond  to  it  in 


MUSCLES   OF   THE    EYE  163 

value,  the  cylinder  will  impair  vision  of  the  line  or  lines  correspond- 
ing to  its  axis. 

If  the  single  line  is  slanted  between  the  parallels  so  that  its 
leftward  end  is  upward,  or  nearer  the  upper  parallel,  while  its  right 
end  is  downward,  or  nearer  the  lower  parallel,  we  may  say  that  its 
left  end  shows  depression  of  the  temple  side  of  the  left  eye,  rela- 


Tar  Q  ^t 

/        


OS ; 0  ji 


0  u 


a  u 


a  u 


FIGURE  34. 

Objective  appearance  in  double-prism  test  of  balance  of  oblique  muscles, 
double  prism  before  right  eye:  1,  target  a  horizontal  line;  2,  as  seen  by  right 
and  left  eyes;  3,  appearance  in  normal  balance  of  obliques;  4,  as  seen  in  infra- 
cyc!o-phoria;  5,  as  seen  in  supra-cyclo-phoria. 

tive  to  the  nasal  side  of  it;  or  depression  of  the  temple  side  of  the 
right  eye,  relative  to  its  nasal  side.  This  is  a  rotation  of  the  eyes 
such  as  to  cause  their  vertical  meridians  to  converge  downward,  or 
diverge  upward.  As  the  fusion  control  is  suspended,  this  indicates 
Infra-cyclo-phoria,  and  therefore  in  the  binocular  fusion  of  the 
images  the  tension  and  strain  falls  upon  the  two  superior  oblique 
muscles,  to  exercise  the  Supra-cyclo-duction  that  is  required  to  cause 
the  images  to  take  the  relative  positions  required  for  their  fusion. 
The  opposite  slant  of  the  single  line  would  indicate  Supra-cyclo- 


164  MUSCLES   OF  THE   EYE 

phoria,  and  require  Infra-cyclo-duction  to  neutralize  it,  or  a  tension 
and  strain  upon  the  two  inferior  oblique  muscles. 

A  horizontal  or  vertical  imbalance  of  the  muscles  can  be  neu- 
tralized by  a  prism,  for  a  simple  prism  has  plane  surfaces,  and 
therefore  neither  converges  nor  diverges  the  rays  of  light,  nor 
directly  affects  or  influences  the  refraction  of  the  eye,  nor  its  focal- 
ization  of  light  from  an  object  at  any  distance.  But,  as  we  can  use 
only  a  cylindrical  lens  to  alter  the  direction  of  a  line,  and  cylinders 
are  required  to  correct  astigmatism,  and  if  not  so  required  they  pro- 
duce artificial  astigmatism,  we  cannot  employ  them  for  the  two  pur- 
poses at  the  same  time,  unless  it  happens  that  the  correction  of 
astigmatism  with  a  pair  of  cylindrical  also  neutralizes  a  cyclo- 
phoria ;  or  the  neutralization  of  a  cyclo-phoria  with  cylinders  having 
oblique  axes  also  corrects  astigmatism,  which  is  very  unlikely  in  one 
case  out  of  a  thousand  or  more.  Apparently  there  is  nothing  we 
can  yet  do  to  correct  cyclophoria,  although  we  can  diagnose  it,  and 
locate  the  muscles  under  strain.  Some  one  may  yet  have  the  happy 
idea  for  the  solution  of  this  problem  in  optometry. 


MUSCLES   OF  THE   EYE  165 

CHAPTER  X 
Dynamic  Phorometry 

In  static  phorometry,  which  naturally  precedes  any  dynamic 
test,  we  endeavor  to  quiet  all  lateral  muscular  activities  or  functions, 
having  or  tending  to  have,  an  inductive  influence  upon  the  particular 
muscles  whose  static  condition,  or  functions,  we  purpose  to  test. 
Therefore  we  use  a  stationary  target,  located  on  the  medial  line  of 
vision  directly  before  the  eyes,  and  at  as  great  a  distance  as  our 
testing  space  will  allow  and  have  the  patient  hold  his  head  erect 
and  stationary.  W''e  also  correct  any  error  of  refraction  found,  to  the 
end  that  vision  shall  be  as  perfect  as  his  acuity  of  vision  will  permit, 
and  the  accommodation  be  inactive.  No  lenses  are  used  except  those 
that  exercise  a  quieting  eflFect  upon  all  of  the  muscles  save  only  those 
that  are  directly  involved  in  the  test  to  be  made,  thus  insuring  a 
correct  static  finding. 

But,  in  dynamic  phorometry,  we  purposely  engage  other  muscu- 
lar activities  and  functions,  for  definite  amounts,  while  making  the 
tests,  and  for  the  purpose  of  determining  the  scope  of  their  influence, 
and  its  effect  upon  the  range  of  the  function  under  examination, 
which  may  operate  more  or  less  fully  when  co-ordinated  with  another 
function  or  acting  in  opposition  to  it.  These  dynamic  methods  are 
also  used  to  develop  or  increase  the  range  of  a  function,  and  neu- 
tralize any  dormancy,  or  tendency  in  that  direction,  of  the  function 
we  are  examining.  The  exercise  of  a  function  in  this  way  tends  to 
free  it  of  influences  that  tend  to  bind  it  down  and  limit  its  range  of 
normal  activities,  and  to  throw  off  temporary  paralysis  or  paresis, 
restore  circulation  of  arterial  blood  to  the  muscle,  and  to  stimulate 
the  discharge,  through  the  lymphatics,  of  effete  matter  that  is  clog- 
ging functional  powers.  But  essentially,  it  is  a  cultivation  of  the 
neuro-motor  control  of  the  muscles  in  the  exercise  of  their  functional 
offices  of  all  kinds. 

Muscle  Tests  for  Near 

A  duction  pair  of  muscles,  apparently  normal  in  a  distance  test 
of  their  balance,  may  still  show  weakness  of  functional  power  in  a 
test  on  a  near  target.  If  the  target  is  still  upon  the  medial  line,  its 
nearness  engages   normal   accommodation   and   adduction.     If   the 


166  MUSCLES   OF  THE    EYE 

eyes  do  not  exercise  due  accommodation  for  the  distance,  vision  of 
the  target  is  sure  to  be  impaired  for  such  near  distance;  and  unless 
adduction  that  is  normal  for  the  distance  is  exercised,  diplopia  will 
result,  homonymous  diplopia  the  same  as  shown  in  exophoria.  But 
we  may  neutralize  the  accommodative  demands  by  plus  lenses  to 
correspond  in  power  to  the  normal  demands  of  such  nearness  upon 
the  accommodation  of  each  eye.  While  these  lenses  eliminate  the 
normal  demand  for  accommodation,  they  do  not  neutralize,  unless 
decentered,  the  normal  demand  for  adduction,  as  it  will  still  be 
necessar)'  for  the  function  of  adduction  to  continue  in  order  to  con- 
verge the  eyes  to  the  near  object,  or  binocularly  to  fix  it.  Otherwise 
there  will  be  diplopia. 

The  effect  of  the  plus  lenses  to  neutralize  the  accommodation 
for  the  nearness  of  the  object,  +3  sphs.  for  13  inches,  unless  de- 
centered  a  considerable  amount,  so  that  the  converged  visual  axes 
pass  through  the  optical  center  of  each,  will  be  to  increase  the  con- 
vergence required,  as  they  will  act  as  prisms,  base  out,  for  such  near 
vision.  These  lenses  will  not  prove  acceptable  to  an  emmetrope  with 
an  ample  fund  of  accommodation,  not  because  the  required  adduc- 
tion will  be  beyond  reach,  but  because  its  inductive  influence  upon 
the  accommodation  will  stimulate  positive  accommodation  in  .spite  of 
the  lenses,  which  would  blur  vision  of  the  near  target.  It  is  here 
that  the  negative  ciliary  fibers  would  be  exercised  to  check  the  action 
of  the  positive  fibers,  and  hold  the  crystalline  lens  to  the  flat  form 
required  to  focalize  the  near  object  through  the  +3  sphs.  It  is 
perhaps  these  counter-activities  that  make  a  pair  of  lenses  of  this 
nature  and  power  uncomfortable  or  intolerable,  except  to  a  pres- 
byope,  whose  accommodation  has  been  weakened  by  age,  and  not 
comfortable  even  to  him  unless  decentered  inward.  If  the  distance  cor- 
rection were  also  plus,  these  effects  would  be  augmented.  In  myopia, 
the  minus  lenses  required  for  a  distance  correction,  would  be  reduced 
in  value  by  the  plus  element  that  neutralizes  the  distance  of  the 
target ;  but  the  adduction,  unless  the  lenses  were  decentered  out- 
ward, would  still  influence  the  accommodation  in  the  same  direction 
and  for  the  same  degree.  There  would  require  to  be  a  balancing  up 
of  the  functions  to  make  near  vision  through  the  distance  correction, 
less  the  .+3  neutralizers,  tolerable. 


MUSCLES   OF   THE    EYE  167 

But  adduction  for  the  near  target  may  also  be  neutralized  by 
prisms,  base  in.  For  a  width  of  6  cm.  between  the  centers  of  rota- 
tion, a  pair  of  9A  prisms  would  be  necessary,  or  a  single  prism  of 
18A,  base  in,  before  one  eye.  Without  the  +3  sphs.  the  adduction 
would  be  neutralized  optically  for  a  target  at  13  inches;  but  the 
eyes  would  have  to  accommodate  3  D.  each  for  the  nearness  of  the 
target,  and  this  would  inductively  excite  adduction,  tending  to  cause 
diplopia.  To  prevent  this  consequence,  which  is  more  intolerable 
than  blurred  near  vision,  the  external  recti  would  be  called  into  func- 
tional action  to  check  such  adduction  as  the  accommodation  incited. 
Clearly,  we  must  neutralize,  in  whole  or  in  part,  both  functions  or 
neither.  But,  to  observe  the  operation  of  these  functional  relation- 
ships, we  must  make  the  muscle  test  for  near,  as  we  have  made  it 
statically  for  distance.  To  do  this  most  effectively,  the  target  should 
be  one  that  is  composed  of  intelligible  characters,  such  as  a  row  of 
letters  in  a  horizontal  line,  with  a  vertical  row  of  letters  crossing 
it,  such  as  the  following : 

B 

FOG 

X 

Fig-  35- 

These  letters  can  be  adapted,  in  size,  to  the  13  inch  distance,  or 
be  of  varied  sizes  for  those  having  greater  or  less  acuity  of  vision. 
A  prism  of  sufficient  power,  base  down  over  either  eye,  will  produce 
vertical  diplopia,  and  we  can  see,  or  the  patient  can  tell  us,  the 
exact  relative  positions  of  the  two  targets  seen.  The  vertical  diplopia 
is  naturally  attributed  to  the  prism,  and  there  is  no  possibility  of 
fusing  the  two  images.  The  words  "BOX"  will  normally  appear 
one  above  the  other,  but  in  direct  vertical  alignment.  If  they  do  not 
assume  this  relative  position,  but  are  separated  in  a  horizontal  direc- 
tion, it  is  proof  that,  without  a  prism  to  account  for  it,  there  is  hori- 
zontal deviation  of  the  visual  axes  from  the  directions  they  are  re- 
quired to  take  to  fix  an  objective  point  at  13  inches.  The  visual 
axes  are  converged  more  or  less  than  the  amount  required  for  fixa- 
tion at  this  distance.     If  the  prism  is  placed  base  down  before  the 


168  MUSCLES    OF   THE    EYE 

right  eye,  the  right  eye  will  see  the  upper,  the  left  eye  the  lower 
target.  We  may  represent  the  above  target  by  a  simple  cross  or 
plus  sign,  -f--  The  different  relative  positions  may  be  represented  as 
follows : 

R.E.        +  +  + 

L.E.        +  4-  + 

Normal         Heteronymy         Homonymy 
Fig.  36. 

What  these  different  effects  mean,  in  a  near  test,  remains  to  be  con- 
sidered. As  diplopia  has  been  produced  by  the  single  prism,  base 
down,  before  the  right  eye,  there  is  no  longer  fusion  control  exer- 
cised. Either  eye  may  fix  the  target  by  merely  directing  visual 
attention  to  it,  in  the  direction  it  appears,  to  that  eye,  to  be.  The 
other  eye,  although  it  does  not  see  the  target  in  that  direction,  will 
follow  it,  though  with  uncertainty,  as  it  cannot  fix  definitely  what 
it  does  not  see  at  all,  except  as  a  sensory  reflex  to  its  foveal  area  of 
fixation.  Both  eyes  together  will  accommodate  for  the  distance, 
although  they  do  not  binocularly  fix  it  in  that  position,  if  they  have 
equal  and  sufficient  power  of  accommodation.  There  will  be  no 
visual  recognition  but  that  both  of  the  eyes  are  fixing  a  single  one 
of  two  targets.  When  vision  is  directed  to  the  other  target,  or  the 
target  as  seen  by  the  other  eye,  both  of  the  eyes  together  turn  toward 
it,  but  only  the  eye  that  sees  it  fixes  it  with  certainty,  the  other  eye 
being  the  uncertain  one;  but  both  eyes  will  accommodate  for  it  as 
before.  If  visual  acuity,  or  accommodative  control,  is  better  in  one 
eye  than  the  other,  the  target  actually  seen  by  the  better  eye  will  ap- 
pear clearer  than  for  the  other.  But  this  is  not  a  circumstance  to  im- 
pair the  muscle  test  for  near. 

Adduction  for  Near 

If,  in  spite  of  the  elimination  of  fusion  control,  by  the  prismatic 
displacement  of  one  of  the  images,  sufficient  to  cause  diplopia,  the 
two  targets  seen  appear  to  be  in  normal  alignment,  the  one  directly 
above  the  other,  then  adduction  for  such  near  distance  is  being  exer- 
cised an  amount  that  is  normal-for-the-distance.     But  this  is  really 


MUSCLES   OF  THE   EYE  169 

an  abnormal  functioning  of  adduction,  since  there  is  no  fusion  stimu- 
lus for  it,  although  the  object  or  target  is  near.  It  will  not  often, 
if  ever,  take  place.  There  are,  however,  two  lateral  incentives  to 
adduction,  although  the  most  important  direct  one  is  taken  away. 
These  are, 

1.  The  accommodation  required  to  be  exercised,  di- 
rectly by  the  fixing  eye  and  indirectly  by  the  other  eye. 

2.  The  visually  observed  and  subjectively  felt  nearness 
of  the  object,  made  manifest  by  other  senses. 

If,  therefore,  in  our  test  at  13  inches,  there  is  heteronymous 
diplopia,  the  upper  target  being  to  the  left  of  the  lower  one,  and  it 
requires  a  6A  prism,  base  in,  to  align  the  upper  and  lower  targets. 
this  shows  that  that  amount  of  adduction  increase  is  necessary  to 
normally  align  them.  If  the  p.  d.  is  6  cm,,  making  normal  adduction 
for  13  inches  18 A,  the  adduction  actually  exercised  is  18  —  6  =  12 A. 
That  is,  notwithstanding  the  elimination  of  the  fusion  stimulus, 
other  influences  cause  adduction  of  2/3  the  normal  amount  to  con- 
tinue to  be  exercised,  or  causes  a  loss  of  but  %  of  the  normal 
amount.  While  from  the  readings  of  the  test  (heteronymous  diplo- 
pia) and  the  fact  that  it  takes  a  prism  value  base-in  to  restore  normal 
positions,  (corresponding  to  exophoria  in  a  distant  test)  it  is  a  little 
anomalous  to  call  it  exophoria-for-near.  Exophoria  is  a  muscular 
abnormality.  This  is  not  an  abnormality,  but  a  natural  result,  some- 
thing to  be  expected  or  anticipated,  under  the  circumstances.  We  are 
surprised  that  there  should  be  so  much  adduction  rather  than  that 
there  isn't  more,  with  the  main-spring  of  adduction  eliminated.  It  is 
apparent  that  the  other  two  influences  are  quite  strong. 

Instead  of  "exophoria-for-near"  the  6 A  that  adduction  falls 
short  of  for  the  distance,  is  an  abatement  of  that  amount,  due  to  the 
want  of  the  normal  stimulus  of  fusion  as  a  controlling  factor.  Its 
resemblance  to  exophoria  is  that  it  takes  a  6A  prism,  base  in,  to  put 
the  targets  seen  in  normal  alignment  for  the  near  distance.  But,  it  re- 
sembles esophoria  also,  for  there  is  a  tendency  of  the  eyes  to  con- 
verge for  near  notwithstanding  the  absence  of  fusion  stimulus.  If 
this  amount  to  12 A,  this  corresponds  to  the  inductive  influence  of 


170  MUSCLES   OF  THE   EYE 

hyperopia,  or  the  accommodation  for  distance  in  hyperopia,  to  cause 
the  eyes  to  converge,  and  is  a  physiological  action.  That  is,  rather  than 
exophoria-for-near,  it  is  physiological  esophoria,  a  convergence  of  the 
eyes  due  to  the  influence  of  normal  accommodation  for  a  near  tar- 
get, as  well  as  to  the  visual  recognition  of  the  actual  propinquity  or 
nearness  of  the  object.  What  we  call  it  does  not  matter  very  much. 
It  will  vary  for  different  patients ;  but  it  throws  an  important  light 
upon  the  functioning  of  the  muscles,  and  the  relationship  of  the 
functions. 

As  the  influence  of  the  accommodation  is  the  principal  cause 
for  adduction  for  near,  under  a  near  muscle  test,  we  may  eliminate 
it,  or  attempt  to,  by  a  pair  of  plus  spheres  for  the  distance  of  the  tar- 
get, in  addition  to  the  distance  correction  of  the  eyes.  This  would  be 
-|-3  spheres  for  a  13"  distance.  As  the  stimulus  for  accommodation 
of  right  and  left  eyes  is  for  targets  in  different  positions,  one  above 
the  other,  these  spheres  should  be  acceptable,  provided  the  target  is 
the  13"  distance  from  them.  That  is,  they  should  relax  all  accom- 
modation, both  of  right  and  left  eye.  and  thereby  eliminate  the 
influence  of  accommodation  upon  adduction.  We  would  then 
naturally  expect  to  find  the  adduction  reduced,  and  the  so-called 
"exophoria-for-near"  increased.  But  what  we  do  find  in  a  particu- 
lar patient  depends  upon  the  personal  equation.  As  the  amount  of 
physiological  adduction  may  vary  for  different  patients,  so  the  effects 
of  relaxing  it  with  spheres  may  vary,  both  for  different  patients  and 
for  the  same  patient  at  different  tests.  As  it  is  very  easy  to  put 
these  spheres  in  a  decentered  position,  thus  increasing  or  decreasing 
the  apparent  adduction,  that  factor  has  to  be  reckoned  with,  as  it  may 
be  misleading. 

The  chief  purpose  of  a  muscle  test  for  near  is  not  to  determine 
the  balance  or  imbalance  of  the  muscles  for  near  with  fusion  elim- 
inated, but  the  functional  activity  or  passivity  of  the  muscles  under 
such  test.  The  more  active  they  are  the  greater  will  be  their  adduc- 
tion or  the  less  their  so-called  exophoria-for-near.  With  the  accom- 
modation eliminated,  there  will  still  be  adduction  of  a  decreased 
amount,  and  in  what  is  then  left  we  have  the  influence  of  mere  pro- 
pinquity of  the  object,  as  both  the  fusion  stimulus  and  the  accommo- 


MUSCLES   OK   THE   EYE  171 

dative  influence  are  eliminated,  or  as  nearly  so  as  we  can  do  it  with 
prisms  and  spherical  lenses  for  the  near  target.  If  there  is  muscular 
paralysis  or  paresis  of  any  of  the  horizontal  muscles,  the  test  will 
show  motor-nerve  control  is  wanting  or  subnormal.  We  do  not  need 
to  stop  with  the  prism  value,  base  in,  that  measures  the  lack  of  adduc- 
tion for  near,  but  increase  the  prism  value  to  a  point  that  carries  it 
over  into  the  homonymous  field,  or  puts  the  upper  target  as  far  to 
the  righl  of  the  lower  one  as  it  is  without  any  prism,  to  the  left  of  it. 
A  prism,  base  in,  will  increase  abduction,  or  abate  adduction,  the 
same  in  a  near  test  as  in  a  distance  test,  and  with  or  without  dip- 
lopia ;  but  it  must  be  placed  over  the  eye  that  is  fixing  the  target  to 
observe  effects.  The  comparison  of  a  near  muscle  test,  or  its  re- 
sults, with  what  we  have  previously  determined  to  be  the  real  mus- 
cular equilibrium  of  the  eyes  by  a  distance  test,  either  confirms  or 
does  not  confirm  the  distance  finding,  and  then  we  may  search  out 
the  cause  of  it  by  a  test  of  the  duction  powers  of  the  muscles  on  a 
distant  target  by  regular  methods.  The  target,  for  near  tests,  should 
be  one  that  engages  the  accommodation  as  previously  suggested. 

Special  Dynamic  Methods 

The  muscular  functions  of  the  eyes  may  be  stimulated  for  a 
distant,  as  well  as  upon  a  near  target,  by  the  use  of  lenses  to  excite 
these  muscular  activities.  As  the  means  of  inciting  accommodation 
for  distance  are  independent  of  those  for  inciting  the  ductions,  there 
is  lacking  in  each  individually  the  co-ordination  that  is  obtained  by 
the  natural  excitation  that  is  produced  on  both  classes  of  functions 
by  a  near  target.  But  by  a  simultaneous  stimulation  of  both,  for 
equivalent  amounts,  an  artificial  co-ordination  of  the  functions  may 
be  brought  about.  It  is  much  simpler,  however,  to  employ  the  nat- 
ural method  of  nearness  than  to  employ  lenses  for  the  purpose  and 
as  it  is  the  method  by  which  the  functions  are  habitually  excited, 
more  reliable  results  are  obtained.  It  will  be  found  convenient  in 
some  instances,  to  combine  the  two  methods  at  the  same  time. 

If  a  pair  of  eyes  are  emmetropic,  either  naturally  or  artificially 


172  MUSCLES   OF   THE    EYE 

so,  minus  spherical  lenses  before  the  eye^  stimulate  positive  accom- 
modation lor  distance.  That  is,  the  lenses  that  correct  myopia, 
when  not  required  for  that  purpose,  make  the  eyes  artificially  hyper- 
opic;  and  the  plus  lenses  that  correct  hyperopia,  when  not  required 
tor  that  purpose,  make  the  eyes  artilicialiy  myopic.  This  is  the  effect 
of  tne  plus  values  that  are  added  to  the  distance  correction  for  the 
relief  of  dehcient  accommodation,  or  the  correction  of  presbyopia, 
for  the  lenses  do  not,  of  course,  supply  the  eyes  with  the  accommo- 
dative power,  whose  deficiency  is  tiie  cause  of  inabihty  to  adapt  the 
eyes  to  near  vision.  Accommodation  may  therefore  be  stimulated 
for  distant  vision  by  minus  lenses  whose  power  is  in  excess  of  actual 
myopia,  or  by  the  reduction  in  power  of  their  plus  correction  for 
distance. 

'the  adduction  lunction  may  also  be  stimulated  for  a  distant 
target  by  a  prism,  base  out,  oetore  either  eye,  or  divided  between 
them.  When  the  eyes  are  in  normal  balance,  ortliopiiona,  a  prism, 
base  out,  excites  adduction  for  the  distant  target,  and  adduction,  hKe 
all  ot  the  ductions,  is  binocular.  It  is  a  function  that  is  engaged  in 
by  a  pair  of  duction  muscles,  never  by  a  single  muscle,  although  but 
one  ot  the  eyes  may  be  required  to  rotate  tor  both  of  them,  it  is 
this  fact  that  many  optometrists  and  refractionists  of  all  kinds  often 
have  a  mistaken  idea  of.  The  refraction  of  the  eyes  is  monocular,  and 
must  be  corrected  in  each  eye  separately ;  but  the  ductions  are  binoc- 
ular, and  the  pair  of  muscles  primarily  involved  are  either  stimu- 
lated or  relaxed  together  by  a  single  prism  before  one  of  the  eyes. 
This  may  be  proved  in  a  simple  way  by  the  prism-and-cover  test,  on 
a  pair  of  eyes  in  normal  balance  in  the  vertical  or  horizontal  direc- 
tions, and  both  objectively  and  subjectively,  and  as  follows: 


Prism  and  Cover  Test 

With  a  distant  small  light  as  a  target  (a  white  card  on  a  dark 
or  contrasting  back-ground  will  answer  the  purpose)  and  with  the 
trial-frame  mounted  for  the  insertion  of  lenses  or  not,  as  necessary, 


MUSCLES   OF  THE   EYE  173 

the  patient  or  subject  binocularly  fixes  the  target  and  sees  it  single. 
Then  the  left  eye  is  covered  by  an  opaque  disc,  the  right  eye  con- 
tinuing to  fix  the  target  alone.  We  then  place  an  8 A,  base  out,  be- 
fore the  right  eye.  The  right  eye  will  turn  to  the  leftward,  or  toward 
the  apex  of  the  prism,  to  continue  to  fix  the  light  in  its  apparently  new 
position.  But  the  left  eye,  under  the  cover,  may  be  seen  to  turn  to 
the  leftward  also,  with  the  right  eye,  although  it  does  not  see  the 
target  or  anything  in  that  or  any  direction.  It  merely  turns  with 
the  right  eye,  making  the  movement  a  leftward  version  of  both  eyes, 
although  the  right  eye  only  is  fixing  the  target  in  the  position  it  ap- 
pears to  be  as  seen  through  the  prism.     Now,  if  the  left  eye"  is  un- 


Tarset 

0 

Co««re 

<1        :              o.H. 

z 

o 

o.  U. 

5 

O              4---      -O 

as. 

:    Cov«i-ocC 

M 

o 

o.  0. 

S 

o 

-->      0 

FIGURE  37. 

Prism  and  Cover  Test:  1,  target  as  binocularly  fixed;  2,  as  seen  by  right 
eye  through  prism;  3,  as  seen  when  left  eye  is  uncovered;  4,  as  seen  by  left 
eye  uncovered;  5,  as  seen  when  right  eye  is  uncovered  with  prism  before  It. 


covered,  it  will  see  an  apparently  new  light,  but  to  the  rightward 
of  the  direction  toward  which  it  is  turned.  This  new  light  will  be 
there  but  momentarily,  but  move  to  the  leftward  and  "fuse"  with 


174  MUSCLES   OF  THE   EYE 

the  other  light.  This  apparent  movement  of  the  "new"  but  really 
original  light  is  due  to  the  rotation  of  the  left  eye  back  to  a  position 
for  fixing  it.  As  the  left  eye  turns  rightward  the  light  will  appear 
to  move  in  the  opposite  direction,  or  leftward,  for  the  left  eye  alone 
sees  it.  The  right  eye  remains  fixed  upon  the  only  light  it  sees,  the 
light  that  is  displaced  by  the  prism. 

To  effect  these  changes  the  left  eye  is  rotated  rightward,  and 
primarily  by  its  internal  rectus  muscle.  The  right  eye,  which  nat- 
urally is  inclined  to  accompany  the  left  eye  in  its  rotation  rightward, 
is  not  allowed  to  do  so,  but  is  restrained  from  so  doing  by  its  inter- 
nal rectus  muscle.  Consequently,  the  muscular  tension  is  put  upon 
the  two  internal  recti  muscles,  simultaneously  and  for  an  equal  divi- 
sion of  the  muscular  labor  involved — the  internal  rectus  of  the  left 
eye  to  rotate  it  back  to  the  position  of  fixation  of  the  target  in  its 
original  position,  and  the  internal  rectus  of  the  right  eye  to  restrain 
it  from  rotating  rightward  with  the  left  eye.  This,  then,  is  merely 
an  adduction  of  the  eyes,  participated  in  by  both  eyes,  although  the 
left  eye  only  makes  the  excursion  of  8A  to  the  rightward.  The  two 
internal  recti  muscles  participate  in  the  adduction  jointly,  not  sev- 
erally, as  is  always  the  case  in  any  duction  of  the  eyes.  Removing 
the  prism  from  before  the  right  eye  causes  both  internal  recti  mus- 
cles to  relax  together.  Covering  and  uncovering  the  right  eye,  be- 
fore which  the  prism  stands,  would  merely  be  a  repetition  of  the 
test,  except  in  changing  from  right  to  left  and  left  to  right  in  the 
description  of  effects. 


Motion  Tests 

When  the  static  tests  have  been  made,  showing  orthophoria  or 
heterophoria  of  a  given  kind  and  amount,  a  dynamic  test  may  be 
made  in  either  of  two  ways : 

I.  By  movements  of  the  target  while  the  face  is  fixed  in  posi- 
tion, so  that  the  eyes  will  rotate  together  to  maintain  fixa- 
tion, or 


MUSCLES   OF   THE   EYE  175 

2.  With  the  target  in  a  fixed  position,  having  the  patient  turn 
the  face  in  different  directions,  causing  the  eyes  to  turn  in 
the  opposite  direction  for  fixation. 

If,  under  such  a  test,  the  two  images  maintain  their  relative 
positions,  both  eyes  are  executing  the  version  together,  although 
the  two  may  not  be  in  the  fusion  positions  for  any  direction.  On  the 
Other  hand,  that  eye  which  sees  its  target  or  image  change  positions 
relative  to  the  other,  or  to  move  more  rapidly  to  right  or  left,  up  or 
down,  is  either  stationary  or  lagging  behind  the  other.  In  this  way 
the  muscle  that  is  not  functioning  normally  is  located. 

Such  a  defect  of  functional  action  cannot  be  corrected  with  a 
prism,  as  it  is  a  weakness  of  functional  action.  A  prism  correction, 
if  attempted,  would  merely  confirm  its  weakness.  What  it  needs  is  a 
course  of  exercise  or  training,  so  as  to  restore  the  weak  function  to 
normal  activity.  Only  static  imbalances  of  the  muscles  may  be  cor- 
rected by  prisms.  Dynamic  inactivities  are  improved  by  muscular 
exercise,  and  this  applies  to  the  versions  as  well  as  the  ductions.  A 
static  imbalance  is  structural  in  character,  so  that  exercise  does  not 
help  it,  or  even  tend  to  overcome  the  imbalance. 


Rotating  Prism  Test 

When  the  eyes  are  binocularly  fixing  a  small  but  distinctive  tar- 
get at  reading  distance  (13"),  normal  accommodation  for  it  is  3  D. 
and  normal  adduction  for  fixation  is  3  meter  angles,  and  from  53^ 
to  6y2  times  as  much  in  prism-diopters,  according  to  the  distance 
between  the  centers  of  rotation,  which  is  practically  the  same  as  the 
pupillary  distance.  For  the  average  width  of  6  cm.  it  is  18 A.  The 
placing  of  a  weak  prism,  base  down  or  up  before  either  eye,  is  suffi- 
cient to  cause  diplopia.  A  5A  prism  will  often  be  sufficient,  but  a 
loA  is  very  sure  to  be.  If  it  is  placed  before  the  right  eye,  base  down, 
that  eye  sees  the  upper  target  of  the  two.  But  the  diplopia  releases 
the  adduction  exercised  for  the  nearness  of  the  target,  and  the  eyes 
usually  diverge  from  it,  causing  the  upper  target  to  appear  to  the 


176  MUSCLES   OF  THE   EYE 

leftward  of  the  lower  one,  which  is  heteronymous  diplopia.  How  far 
the  upper  target  appears  to  move  to  the  leftward  of  the  lower  one 
depends  on  how  fully  the  internal  recti  muscles  relax,  or  adduction 
is  abated.  The  prism  has  produced  only  vertical  diplopia,  so  that  it 
accounts  only  for  vertical  separation.  The  horizontal  separation 
comes  from  physiological  activities  or  inactivities  of  the  muscles. 

To  make  the  rotating  prism  test,  the  prism  (loA)  should  be 
in  the  forward  cell  of  the  3-cell  trial  frame.  This  provides  a  place 
for  such  sphere  or  cylinder  as  may  be  required  to  correct  the  refrac- 
tion of  either  eye,  but  the  cylinder,  if  any  is  required,  must  be  placed 
in  the  back  non-rotating  cell.  The  handle  of  the  prism  may  be  bent 
outward  so  as  not  to  interfere  with  its  rotation  in  the  frame.  The 
target  for  this  purpose  should  be  a  small  cross,  its  bars  being  vertical 
and  horizontal  to  enable  the  patient  to  clearly  report  effects.  To 
make  a  distinct  separation  of  the  two  targets  seen  at  this  distance, 
the  vertical  bar  should  not  be  longer  than  3  cm.  and  may  be  much 
shorter  provided  it  is  distinct  for  the  distance.  As  the  fusion  control 
is  quieted  by  the  diplopia,  there  will  be  no  muscular  action  to  dis- 
turb the  apparent  positions  of  the  targets  as  seen.  But  a  rotation  of 
the  prism  in  either  direction  will  displace  the  upper  target  in  the 
direction  the  apex  of  the  prism  turns.  We  may  turn  it  in  either 
direction,  but  it  must  be  turned  in  the  direction  of  base  in  to  bring 
the  upper  and  lower  targets  into  alignment. 

Slowly  rotating  the  prism  in  its  cell  iq  this  direction,  its  apex 
is  successively  at  105°,  120°,  135*,  150°,  165°  and  180°,  as  well  as  at 
all  intermediate  positions.  As  the  rotation  proceeds  the  two  targets 
approach  each  other,  both  vertically  and  horizontally,  for  the  prism, 
by  the  rotation,  is  gradually  acquiring  power,  base  in,  and  as  grad- 
ually losing  power,  base  down.  At  120°  it  has  75^  A  power,  base 
down,  or  has  lost  2^ A  of  its  original  loA  in  that  direction;  but  it 
has  acquired  the  25^  A  power,  base  in.  At  135°  it  has  5  A  base  down 
and  5  A  base  in,  or  what  it  loses  in  one  direction  it  acquires  in  the 
other.  By  the  downward  and  rightward  movement  of  the  upper  tar- 
get it  will  eventually  arrive  at  the  position  directly  above  the  lower 
one,  putting  the  vertical  bars  in  vertical  alignment,  but  the  horizontal 


+ 

4- 

+ 

+ 

+ 

+ 

+ 

3- 

4- 

5- 

6. 

MUSCLES   OF  THE   EYE  177 

bars  still  separated  vertically.    These  changes  referred  to,  and  fur- 
ther ones,  are  indicated  by  the  following: 

Right:     + 

+ 
Left :  +  + 

I.  2. 

At  I  the  targets  are  quite  widely  separated,  the  vertical  separa- 
tion being  due  to  the  prism  action ;  the  horizontal  heteronymous  sepa- 
ration showing  merely  that  without  fusion  control,  less  than  normal 
adduction  for  the  distance  of  the  target  will  result.  But  the  distance 
of  horizontal  separation  will  vary  for  different  persons.  At  2  the 
apex  of  the  prism  is  perhaps  at  120°,  and  this  is  due  to  the  power  of 
the  prism  being  oblique,  ^4  down  and  34  in.  At  3  the  targets  are  in 
alignment,  the  upper  directly  above  the  lower,  but  only  half  of  the 
original  vertical  separation  and  no  horizontal  displacement.  This 
result  will  be  in  the  neighborhood  of  axis  135°,  for  there  is,  at  that 
point,  still  5 A  base  down,  and  that  is  too  much  usually  for  sursum- 
duction  to  overcome.  There  is  no  necessity  for  adduction  to  be  exer- 
cised at  all,  for  the  targets  are  in  vertical  alignment.  This  indicates 
that  the  adduction  due  to  otlier  influences  than  fusion  are  effective 
for  about  13 A,  while  the  5 A,  base  in,  exercised  by  the  prism  does 
the  rest.  But  this  factor  is  a  varying  one.  At  4  the  upper  target  has 
passed  over  to  the  other  side  of  the  lower  one,  and  is  but  slightly 
above  it.  The  next  movement  will  bring  it  within  the  range  of  sur- 
sum-duction,  and  that  duction  will  make  horizontal  adjustment  possi- 
ble, so  that  fusion  takes  place  between  4  and  5  or  5  and  6,  and  there 
is  only  the  one  target  to  be  seen. 

With  all  lateral  factors  eliminated,  so  that  the  functioning  of 
the  accommodation  for  the  distance  of  the  target  and  the  gradual 
shifting  of  the  prism  power  from  base  down  to  base  in  are  alone  in- 
volved as  dynamic  factors,  the  assumption  of  the  relative  positions 
of  the  target  shown  in  number  3,  or  in  vertical  alignment,  when  the 
apex  of  the  prism  is  at  or  near  135°,  indicates  normal  adduction  for 
near  under  these  dynamic  influences.  If  fusion  of  the  images  takes 
plac«  when  the  apex  of  the  prism  reaches  150°,  or  thereabouts,  this 


178  MUSCLES   OF  THE   EYE 

indicates  normal  functioning  of  all  of  the  recti  muscles  involved,  both 
horizontally  and  vertically.  The  initial  position  of  the  upper  target 
will  vary  somewhat  for  different  persons  and  that  may  be  considered 
as  modifying  the  above,  but  only  slightly.  If  a  5A  prism  is  used, 
fusion  should  take  place  at  number  3,  for  only  2^  A  of  sursum- 
duction  would  then  be  necessary  to  bring  the  upper  target  down  to 
the  lower  one,  while  an  equal  amount  of  abduction,  or  the  abatement 
of  that  amount  of  adduction,  due  to  other  influences  than  fusion  con- 
trol, would  bring  the  two  targets  together  horizontally,  or  "fuse"  the 
two  images.  This  would  reduce  the  adduction  from  13 A  to  11 5^  A 
for  the  13"  distance  of  the  target,  which  may  be  done  without  com- 
promising the  accommodation,  or  if  it  does,  relax  it,  as  it  would  tend 
to  do.  These  tests  are  principally  important  in  affording  the  optom- 
etrist the  opportunity  to  observe  the  workings  and  associations  of  the 
functions. 

Dynamic   Skiametry  Test 

This  is  really  a  test  of  the  refraction  of  the  eyes  while  they  are 
accommodating  and  converging  for  a  near  target,  but  shows  the  rela- 
tivity or  potentiality  of  the  associated  functions.  The  usual  methods 
of  applying  this  test  are  to  have  the  distance  of  the  operator,  and 
mirror,  the  same  as  that  of  the  target,  or  the  target  attached  to  the 
mirror,  and  to  shadow  test  the  eyes  while  they  are  converging  and 
accommodating  for  it.  Then  target  and  mirror  are  both  brought 
nearer  or  farther  from  the  eyes  being  shadow  tested  by  a  movement 
of  the  operator  to  a  nearer  or  farther  position.  It  is  assumed  that 
in  converging  for  the  near  target,  a  co-ordinate  degree  of  accommo- 
dation will  be  exercised,  and  that  a  plus  lens  or  pair  of  them  will 
not  relax  it.  except  accommodation  in  excess  of  the  normal  amount 
for  the  distance.  Therefore,  whatever  plus  lens  may  be  necessary 
to  neutralize  the  shadow  movement  is  a  measure  of  such  excess  ac- 
commodation, and  therefore  of  the  hyperopia.  It  brings  the  excess 
out  by  loading  the  function  of  accommodation  down  with  more  than 
it  can  do,  and  so  reveals  the  excess,  or  makes  it  manifest  by  the 
shadow  movements  thus  uncovering  the  latent  hyperopia. 


MUSCLES    OF   THK    EYK 


179 


In  the  method  we  describe  this  plan  is  varied.  The  operator  takes 
any  convenient  distance  from  the  patient,  i  meter,  2/3  meter,  ^ 
meter,  or  even  a  nearer  distance,  according  to  the  clearness  with 
which  he  can  observe  the  reflex  movements.  He  provides  himself 
with  a  small  target,  about  the  size  of  a  trial  case  lens,  mounts  it  upon 
a  handle  similar  to  that  of  the  hand  retinoscope,  having  the  small 
letters  or  characters  on  one  side  of  it  and  the  dots  to  be  counted  upon 
the  other.  Holding  the  target  before  his  left  eye.  the  patient  is  asked 
to  count  the  dots  or  to  read  the  small  letters  upon  it,  and  while  he  is 
doing  so  the  optometrist  quickly  shadow  tests  each  eye.  The  target 
thus  serves  as  a  "blind"  for  the  left  eye  as  soon  as  he  has  made  the 
reflex  appear  in  the  patient's  pupil,  li  in  this  position  motion  is  zinth 
the  plain  mirror,  he  knows  that,  although  endeavoring  to  accommo- 
date for  the  distance,  under  the  urge  of  an  equal  degree  of  adduction, 
the  patient  is  not  accommodating  the  normal  amount  for  the  target. 
He  must  converge  for  it  to  prevent  diplopia,  but  accommodation  is 
less  imperative  in  its  demands,  so  the  accommodation  lags,  or  is  less 
than  the  equivalent  convergence.  Instead  of  inserting  glass  lenses 
before  the  eye  to  neutralize  motion,  a  simpler  and  more  convenient 
method  is  used,  as  follows : 

The  target  is  moved  toward  the  patient  while  the  operator  re- 
mains at  his  fixed  distance.  This  he  does  by  simply  carrying  it  for- 
ward, toward  the  patient.     The  small  letters  and  dots  make  larger 


FIGURE  30. 


Dynamic  Skiametry  Test.  Working  distance  of  operator,  1  meter.  Target 
at  27"  shows  neutral  band  vertically;  target  at  20"  shows  neutral  band  hori- 
zontally; lens  before  eye.  +2  sph.  Results:  add  +.50  to  vertical  and  +1.00  to 
horizontal,  making  Rx  +  2.50  sph.  C  +  -50  cyl.  ax.  90. 

L,  source  of  light;  M,  mirror;  ~,  operator's  eye;  P,  patient's  eye  at  1  meter; 
T,  target  at  27"  from  P;  vertical  neutralization;  T',  target  at  20"  from  P, 
horizontal  neutralization. 


180  MUSCLES   OF   THE    EYE 

images  upon  the  retinae,  although  they  may  be  focaUzed  less  per- 
fectly than  at  the  operator's  working  distance;  but  the  patient  will 
converge  to  the  nearer  distance  and  endeavor  to  increase  his  accom- 
modation. That  the  combined  action  (accommodation  and  conver- 
gence) are  effective  is  shown  by  the  fact  that  in  this  way  the  shadow 
movements  are  neutralized  and  may  be  made  to  move  agaiftst  the 
mirror.  With  the  subjective  effects  upon  the  vision  of  the  patient, 
his  vision  of  the  target  at  the  nearer  distance,  we  are  not  interested, 
except  as  a  means  of  enforcing  increased  accommodation,  or  in  mak- 
ing the  accommodation  supply  the  increased  plus  required  to  neu- 
tralize the  shadow  movements  in  the  pupils,  the  objective  effects. 
That  the  method  is  effective  in  doing  so  is  proved  by  the  fact  that 
with  the  advancement  of  the  target  the  point  of  reversal  in  both  eyes 
advances,  and  soon  shows  neutral  reflex  motion  in  the  pupil  of  one 
or  both  eyes.  Assuming  that  the  working  distance  is  i  meter,  the 
target  may  have  to  be  moved  forward  to  a  distance  of  27  inches 
(66  cm.),  or  to  a  position  20""  (50  cm.)  or  even  to  16",  13"  or 
10"  (the  equivalent  of  40  cm.,  33  cm.  or  25  cm.)  from  the  patient  to 
bring  this  about.  We  have  thus  enforced  an  accommodative  in- 
crease of  the  dioptric  differences  between  the  position  of  the  target 
and  the  working  distance  of  the  operator. 


Dioptric  Equivalents, 

The  equivalents,  in  dioptric  increase  of  accommodation,  for 
these  distances  of  the  target,  or  for  any  distance  of  it  required,  are 
readily  worked  out.  If  the  working  distance  is  i  meter  or  40  inches, 
and  the  operator  and  target  are  in  that  position,  but  motion  of  the 
reflex  or  shadows  is  with  the  mirror,  the  target  is  advanced,  thus 
engaging  co-ordinate  action  of  the  functions  of  accommodation  and 
adduction  or  inciting  them  to  action.  For  the  respective  positions 
the  dioptric  values  of  the  increase  in  accommodation  are  respectively 
as  follows: 

1.  For  target  at  40",  neutral  motion,  no  increase,  emmetropia 

2.  For  target  at  32",  neutral  motion,  .25  D.  increase  hyperopia 


MUSCLES    OF   THE    EYE  181 

3.  For  target  at  27",  neutral  motion,    .50  D.  increase,  hyperopia 

4.  For  target  at  23",  neutral  motion,    .75  D.  increase,  hyperopia 

5.  For  target  at  20",  neutral  motion,  i.oo  D.  increase,  hyperopia 

6.  For  target  at  16",  neutral  motion,  1.50  D.  increase,  hyperopia 

7.  For  target  at  13",  neutral  motion,  2.00  D.  increase,  hyperopia 

The  method  of  reducing  to  dioptric  equivalents  is  merely  to  re- 
duce the  target  distance  from  the  patient's  eyes  to  its  dioptric  equiva- 
lent, and  from  it  take  the  dioptric  equivalent  of  the  operators  work- 
ing distance.  The  difterence  thus  obtained  represents  the  probable 
amount  of  latent  hyperopia,  or  hyperopia  that  is  concealed  from 
ordinary  subjective  or  even  objective  discovery. 

It  may  be  questioned  how  the  inciting  of  the  accommodation  to 
greater  activity  in  this  way  can  possibly  bring  out  a  latent  element, 
for  an  increase  of  accommodation  may  be  said  to  be  "on  top"  of  that 
previously  exercised,  while  a  latent  element,  due  to  spasm,  is  at  the 
bottom  of  it — the  part  that  fans  to  relax  or  respond  to  plus  lenses. 
That  is  a  question  in  philosophy,  and  neither  the  originator  of  this 
method,  nor  the  writer  set  up  as  philosophers.  Perhaps  the  latent 
element  filters  up  through  the  body  of  the  manifest,  as  cream  rises 
on  a  pan  of  milk  after  all  of  it,  apparently,  has  been  skimmed  off. 
Results  are  what  count  in  practice,  and  it  is  certain  that  it  is  the 
objective  signal  of  the  shadow  movements  at  the  eye  of  the  observer 
that  shows  what  the  state  of  refraction  of  the  eye  being  examined  is, 
wherever  the  target  he  is  trying  to  see  may  be  located,  and  whether 
he  sees  it  clearly  or  not.  If  there  is  a  real  myopia,  greater  than  the 
dioptric  equivalent  of  the  working  distance,  the  shadow  movements 
will  be  against  the  plane  mirror.  A  part  of  this  may  be  neutralized 
by  taking  a  nearer  working  distance;  then,  by  carrying  the  target 
farther  back,  provide  the  stimulus  for  less  adduction  and  accommo- 
dation, by  that  means  reducing  the  minus  value  indicated  for  the  cor- 
rection of  the  real  myopia.  The  dioptric  equivalent  of  the  working 
distance,  in  plus,  will  then  be  greater  than  that  of  the  target  distance, 
and  the  subtraction  of  the  former  from  the  latter  will  give  the  minus 
result.  That  is,  a  2  D.  working  distance  (20'')  from  a  i  D.  target 
distance  (40")  is  i — 2  =  — i  D.,  the  correction. 


182  MUSCLES    OF   THE    EYE 

This  is  but  a  brief  outline  of  the  method.  It  appears  to  have 
been  originated  by  Joseph  Smith,  optometrist,  of  Cambridge,  Ohio. 
But  his  later  collaborator,  O.  L.  Altenberg,  of  Toledo,  Ohio, 
has  devised  some  interesting  means  of  applying  the  method  and  as- 
sisted in  its  development.  There  are  apparently  a  number  of  optom- 
etrists who  have  been  persuaded  to  try  the  method,  and  those  who 
could  do  so  efficiently  liave  found  it  gives  most  satisfactory  results.  It 
is  especially  valuable  in  astigmatic  cases,  as  the  different  positions  of 
the  target  at  once  give  successively  its  neutralizing  position  for  the 
principal  meridians,  and  a  clearly  marked  location  for  the  axis  of  the 
cylindrical  correction.  When  shadow  movements  are  slow  and  indis- 
tinct, a  supplementary  lens  may  be  employed  to  bring  the  point  of 
reversal  nearer  to  or  forward  of  the  observer's  eye,  thus  accentuating 
them,  or  making  the  results  more  clearly  visible  to  the  operator.  If 
the  reader  will  bear  in  mind  that  this  is  a  method  of  determining  the 
full  ametropia  of  the  eyes  by  objective  means,  and  not  to  enable  the 
patient  to  see  at  any  distance,  and  that  the  eye  is  refracted  for  the 
bright  fundus  reflex,  and  not  for  the  target  at  all,  he  may  get  great 
advantage  from  its  use,  and  perhaps  become  wedded  to  it  as  a  quick 
and  reliable  method  for  measuring  ametropia,  as  those  who  now 
practice  it  declare  it  to  be. 

It  may  appear  to  the  superficial  observer  that  dynamic  methods 
that  are  not  effective  in  arousing  a  desired  action  therefore  fail,  but 
this  is  not  so.  If  the  purpose  is  to  stimulate  a  muscular  action,  or  to 
relax  it,  the  method  succeeds  although  that  effect  fails.  In  presby- 
opia, one  is  unable  to  get  the  accommodative  mechanism  to  be  ef- 
fective in  convexing  the  lens  beyond  its  limit ;  but  the  putting  forth 
of  an  ineffective  muscular  action,  if  that  is  the  purpose,  is  successful, 
even  though  it  is  ineffective  in  convexing  the  lens.  In  all  of  the  duc- 
tions,  the  muscular  action  is  involuntary,  and  even  that  may  result  in 
merely  stabilizing  the  eyes,  or  one  of  them,  in  its  position.  If  the 
eyes  are  fixing  a  target  at  any  distance,  and  a  prism  is  placed  before 
one  of  them,  that  eye  over  which  the  prism  is  placed  rotates  to  a 
new  position.  It  has  to,  to  continue  to  fix  the  target  as  seen  through 
the  prism.    But  the  other  eye  remains  fixed  as  it  was,  for  it  also  has 


MUSCLES   OF   THE    EYE  183 

to  do  that  to  continue  to  fix  the  object  or  target.  Hence,  there  is 
muscular  contraction  of  both  muscles  of  the  duction  pair  involved, 
although  but  one  of  the  eyes  rotates.  So,  in  all  dynamic  methods,  it 
is  not  the  effectiveness  in  producing  a  motion  that  measures  the  direc- 
tion and  amount  of  the  action,  but  the  neuro-motor  stimulus  that 
the  muscle  gets,  whether  to  rotate  or  effect  a  movement,  or  to  pre- 
vent one,  or  to  restrain  it.  Muscles  act  as  "checks"  to  movements 
as  well  as  to  produce  movement ;  and  where  there  is  a  movement 
possible  in  any  direction,  there  is  a  check  and  counter  movement  to 
restrain  the  first  or  produce  the  opposite  movement,  and  this  is  as 
true  of  the  accommodation  and  ciliary  action  as  of  any  other. 

It  is  possible  that  the  discrepancy  between  the  subjective  focali- 
zation  of  the  target  for  vision  at  the  nearer  distance,  and  the  objec- 
tive focalization  of  the  fundus  reflex  at  the  observer's  eye,  may  be 
due  to  the  fact  that  the  former  is  focalized  at  the  rods  and  cones  of 
the  retina,  at  its  posterior  surface ;  while  the  light  that  is  focalized 
objectively  at  the  observer's  eye  is  from  the  anterior  surface  of  the 
retina,  and  slightly  nearer  the  dioptric  media,  which  would  carry  its 
conjugate  to  a  greater  distance  from  the  eye.  The  retina  is  very  thin 
but  there  are  eight  layers  of  nervous  tissue  between  the  two  surfaces, 
so  that  a  separation  of  even  this  small  amount  is  in  favor  of  putting 
the  conjugate  of  the  anterior  surface  farther  forward,  or  nearer  the 
eye  of  the  observer  than  that  of  the  rods  and  cones,  the  sensory 
visual  field. 

If  that  is  the  explanation  of  it,  then  by  shadow  testing  we  re- 
fract the  eye  for  the  non-sensitive  anterior  surface  of  the  retina, 
instead  of  its  sensory  field  of  vision.  The  red  that  shows  through  in 
the  fundus  reflex  is  due  to  the  transparency  of  the  retina,  as  the 
vascular  elements,  the  arteries  and  veins,  are  even  farther  back  in 
the  structure.  Perhaps  very  few  will  ever  discover  or  appreciate 
such  a  slight  difference  as  might  be  made  by  so  thin  a  layer  of  tissue 
as  that  of  the  retina,  but  very  small  distances  are  highly  potential 
near  the  focus  of  a  lens  of  any  kind ;  and  the  nearer  they  are  to  it 
the  more  potential  they  are,  especially  in  a  high  power  lens  like  that 
of  the  dioptric  media  of  the  eye,  A  distance  of  1/17  of  a  focal 
length,  or  i  mm.,  makes  a  difference  of  17  focal-lengths  in  its  con- 


184  MUSCLES   OF   THE   EYE 

jugate,  or  289  mm.  =  about  11.5  inches  from  the  anterior  principal 
focus.  But  the  thickness  of  the  retina  is  very  much  less  than  i  mm., 
and  therefore  its  conjugate  is  placed  at  a  greater  distance.  For 
.25  mm.  it  would  be  practically  i  meter. 


MUSCLES    OF   THE   EYE  185 

CHAPTER  XI. 
Muscular  Exercise. 

If  you  see  a  young  man,  fresh  from  his  morning  bath,  holding 
a  pencil  or  other  small  object  at  reading  distance  from  his  eyes  and, 
while  his  head  is  rigid,  moving  this  target  from  right  to  left  and  up 
and  down,  or  around  in  circles,  while  his  eyes,  to  maintain  fixation 
of  it,  rotate  in  their  orbits ;  and  a  moment  later  he  holds  the  target 
in  a  fixed  position  and  makes  the  gyrations  with  his  head,  but  main- 
tains binocular  fixation  of  the  target  by  rolling  the  eyes  in  their  or- 
bits, you  may  say  that  he  is  a  disciple  of  good  old  Dr.  Taylor  (peace 
to  his  memory)  of  South  Dakota,  who  made  a  hobby  of  "oculo- 
didactics"  and  taught  the  system  to  the  pupils  of  many  public  and 
private  schools  in  his  own  and  neighboring  states.  In  his  viewpoint, 
or  from  it,  these  exercises  strewed  the  path  of  Hfe  of  the  one  who 
practiced  them  daily  with  roses,  cultivated  alertness  of  movement, 
poise  and  grace  of  posture  and  motion,  not  only  of  the  eyes  but  of 
the  entire  body  as  well. 

As  an  optometrist,  he  appealed  with  some  authority  to  school 
superintendents  and  teachers.  He  had  a  pleasing  philosophy  on  the 
subject  and  would  discourse  upon  it  in  an  interesting  way,  as  most 
people  with  a  hobby  are  able  to  do.  In  later  life  he  often  entertained 
optical  conventions  with  a  discourse  upon  his  methods,  and  the  bene- 
fits to  be  derived  from  a  system  of  "muscular  exercise"  of  this  kind, 
for  quiet  and  steady  eyes  are  symbolical  of  a  quiet  and  steady  mind ; 
and  alertness  and  quickness  of  movement  of  the  eyes,  and  the  culti- 
vation of  these  powers,  are  reflexed  to  all  the  muscles  of  the  body. 
On  the  other  hand,  any  deformity  of  bodily  movement  or  posture, 
such  as  a  high  shoulder  or  a  slanting  hip,  or  a  wobbling  walk  or  awk- 
ward sitting  position,  indicated  the  need  of  ocular  muscle  exercises. 
They  tend  to  stiffen  the  flabby  muscles  and  to  reUeve  the  rigidity  that 
muscles  acquire  from  non  use.  He  has  many  disciples  among  optom- 
etrists today,  and  their  numbers  are  apparently  increasing.  Even 
the  president  emeritus  of  the  national  association  is  one  of  them. 

Methods  of  Exercise 

There  are  two  principal  methods  of  stimulating  the  ocular  mus- 
cles to  action,  or  of  relaxing  them  to  the  fullest.    Optometry  affords 


186  MUSCLES   OF   THE   EYE 

every  opportunity  to  use  either  or  both  methods,  or  to  combine  them. 
Briefly,  these  methods  are  as  follows : 

1.  The  Natural  Method. 

(a)  While  the  patient  is  fixing  the  target  at  reading  dis- 
tance, it  is  moved  from  side  to  side,  up  and  down,  and  circled 
or  gyrated  about. 

(b)  While  the  target  is  stationary,  the  patient  makes  the 
gyrations  with  his  head,  but  maintains  fixation  of  the  stationary 
target. 

2.  The  Artificial  Method. 

(a)  While  head  and  target  are  in  fixed  positions,  lenses  to 
relax  or  stimulate  specific  muscular  action  are  placed  before  one 
or  both  eyes. 

(b)  Prismatic  discs  that  cause  an  optical  displacement  of 
the  target  are  placed  before  one  or  both  eyes,  and  their  values 
progressively  increased  or  reduced. 

The  natural  method  alone  affords  a  quite  wide  scope  for  mus- 
cular exercise,  for  by  changing  the  direction  of  the  target  the  ver- 
sion pairs  of  muscles  may  be  successively  called  into  play,  or  stimu- 
lated to  action;  and  by  changing  its  distance  from  the  eyes,  adduc- 
tion or  abduction,  along  with  increased  or  reduced  accommodation,  is 
brought  about.  A  distant  target  could  not  be  easily  changed  in 
"direction"  from  the  eyes,  so  as  to  stimulate  the  versions,  but  the 
patient's  head,  under  the  optometrist's  direction  or  guidance,  is  easily 
manipulated  as  required,  to  call  for  any  of  the  versions.  But,  since 
the  limited  ductions  that  can  be  exercised  by  the  natural  method  re- 
quire a  near  target,  the  version  exercises  may  also  be  exercised  for 
it.  Let  us  consider  a  simple  method  for  the  latter  exercise  of  the 
muscles  with  both  the  direction  and  distance  of  the  target  under 
control. 

The  Natural  Method 

This  requires,  for  the  near  distance,  a  distinctive  target  that  is 
visible  at  reading  distance  and  for  some  distance  farther  away.  Hold- 
ing it  at  reading  distance  before  the  patient  and  asking  him  to  read  it. 


MUSCLES   OF   THE    EYE  187 

he  at  once  exercises,  if  he  can,  the  required  adduction  to  fuse  the 
images  on  the  retinae,  and  the  accommodation  required  to  make  tlie 
images  as  clear  as  possible.  As  he  may  be  slightly  presbyopic,  or  have 
less  than  normal  acuity  of  vision,  difi'erent  sizes  of  letters  for  differ- 
ent classes  of  patients  should  be  provided.  If  he  sees  and  reads  the  let- 
ters on  the  target  card,  he  is  assumed  to  be  adducting  and  accommo- 
dating normally  for  the  distance,  although  he  may  not  be  quite 
reaching  the  normal  mark  in  accommodation.  If  there  is  any  faint- 
ness  to  his  vision  at  this  distance,  the  target  may  be  moved  from  13 
to  16  or  even  to  20  inches.  The  demand  for  adduction  is  thereby 
abated,  and  abduction  turns  the  visual  axes  outward. 

Assuming  that  he  sees  it  clearly  and  easily  at  20  inches,  this 
position  represents  accommodation  of  2  D.  by  each  eye,  and  about 
12 A  of  adduction.  If  the  target  is  on  the  medial  line  of  vision,  there 
is  normally  no  version  exercised.  We  then  direct  the  patient  to  hold 
his  head  firmly  in  that  position,  or  not  to  turn  it  in  any  direction. 
The  target  is  then  moved  as  far  upward  as  he  is  able  to  see  it ;  then 
as  far  downward  and  then  to  the  right  and  left.  As  soon  as  the 
patient  understands  exactly  what  you  want  him  to  do,  he  will  do  it 
readily.  You  then  gyrate  the  target  through  various  curves,  moving 
it  first  in  one  direction  and  then  in  another,  but  maintaining  the  dis- 
tance of  20  inches,  or  plane  of  a  20  inch  distance.  After  a  brief 
exercise  of  this  kind,  you  ask  him  to  look  at  a  target  with  finer  let- 
ters upon  it,  and  bring  it  nearer  to  the  eyes ;  and  one  with  coarser 
letters  and  carry  it  farther  from  them.  This  stimulates  successively 
positive  and  negative  accommodation,  and  with  it  adduction  and 
abduction  of  the  eyes. 

Having  determined  his  range  of  good  single  vision,  a  medium 
target,  or  one  with  the  medium  sized  letters  upon  it,  may  be  given  all 
of  these  different  movements,  up  and  down,  to  right  and  left,  round 
and  around,  and  brought  nearer  to  the  eyes  and  moved  to  a  greater 
distance  from  them,  thus  caUing  into  play  all  of  the  recti  muscles, 
both  in  version  and  duction  pairs.  It  isn't  necessary  to  go  to  the 
limit  in  any  of  these  movements.  Comparatively  slight,  but  more 
and  more  rapid  ones,  are  preferable.  A  two  or  three  minute  period 
of  this  exercise  will  be  tiring  at  first  to  both  operator  and  patient, 


188  MUSCLES   OF  THE   EYE 

but  later  to  the  patient  only ;  and  he  will  soon  be  able  to  give  him- 
self the  exercise  without  assistance,  but  don't  neglect  to  see  that  he 
does  it  under  your  observation.  To  vary  the  exercise  stabilize  the 
target  at  the  vision  distance,  and  have  the  patient  make  correspond- 
ing movements  of  the  head,  while  he  approaches  and  recedes  from 
the  fixed  target.  In  this  phase  of  the  exercise,  neck  and  eye  muscles 
are  simultaneously  employed. 

There  seems  to  be  no  objective  or  natural  way  of  stimulating 
the  oblique  muscles,  except  in  co-ordination  with  the  recti  muscles, 
nor  any  way  of  observing  or  measuring  the  amount  of  their  action. 
The  eyes  have  no  markings  to  indicate  whether  the  vertical  merid- 
ians are  vertical  or  the  horizontal  meridians  are  horizontal.  In 
appearance  they  are  pretty  much  alike  all  the  way  around,  or  in  all 
directions  from  the  pupils.  It  would  not  be  perceptible  outwardly 
if  one  or  both  eyes  were  turned  upon  their  anterio-posterior  axes  5° 
or  20°,  in  the  same  or  opposite  directions,  for  they  would  still  have 
apparently  the  same  front.  But,  such  a  rotation  of  the  eyes  would 
rotate  the  retinae  the  same  amounts,  which  would  manifest  itself 
visually  or  subjectively.  We  therefore  have  to  depend  upon  arti- 
ficial methods  for  exercising  these  muscles,  and  can  only  estimate 
the  amount  of  the  torsions  or  cyclo-ductions  from  subjective  effects 
that  are  entirely  relative. 

Artificial  Methods 

These  consist  of  the  employment  of  material  agents  of  any  kind 
to  interfere  with,  modify  or  alter  the  course  of  light  in  its  propaga- 
tion from  the  object  to  the  eyes,  such  agents  being  placed  directly 
forward  of  either  or  both  eyes,  so  as  to  allow  the  light  from  the  dis- 
tant target  to  take  its  natural  course  up  to  that  point.  We  may,  in 
this  manner,  artificially  reduce  the  volume  of  light  entering  the  eye, 
or  confine  it  to  one  meridian,  as  with  the  pin-hole  or  stenopaic  discs, 
and  this  will  affect  the  functioning  of  the  muscles  of  the  iris.  We 
may  use  spherical  or  cylindrical  lenses  to  affect  the  normal  focaliza- 
tion  of  light  upon  the  retinae,  and  thereby  stimulate  or  quiet  mus- 
cular action  of  the  ciliary  muscles.  We  may  cause  two  entirely  dif- 
ferent images  to  appear  upon  the  two  retinae,  and  so  deviate  the 


MUSCLES   OF  THE   EYE  189 

course  of  light  to  one  or  both  of  them  as  to  cause  them  to  occupy 
corresponding  positions,  thus  enabhng  the  visual  sense  by  their  re- 
lated character,  to  fuse  them  into  one,  or  to  see  two  targets  as 
though  it  were  but  one.  We  can  so  displace  one  or  both  images  that 
they  would  be  separated,  thereby  making  the  eyes  see  one  object 
as  two,  and  thereby  stimulate  such  action  of  the  extrinsic  muscles 
as  will  place  the  retinae  in  the  relative  positions  required  for  fusing 
the  two  images  into  one,  or  to  cause  fusion  of  the  images. 

The  appeal  is  made  directly  to  the  visual  sense,  in  all  of  these 
means  of  altering  the  normal  or  natural  course  of  action ;  and  our 
object  is  either  to  allay  or  subdue  muscular  action  of  some  char- 
acter by  special  muscles,  or  it  is  provocative  of  such  action  by 
them.  In  this  way  we  exercise  the  muscles  without  altering  the 
position  of  the  target  or  changing  its  character.  We  know  that  the 
visual  sense  will  "take  alarm"  at  any  interference  with  normal  see- 
ing, and  at  once,  through  its  voluntary  or  involuntary  motor-nerve 
agents,  stimulate  the  muscular  action  that  is  required  to  neutralize, 
as  far  as  possible,  such  interference.  Muscular  exercise  is  the  alter- 
nation of  contraction  and  relaxation  of  a  muscle,  not  its  steady  con- 
traction, nor  its  contraction  to  the  limit  of  its  power.  Hence,  by 
using  low  power  agents  for  interference,  and  removing  them  quickly, 
a  wide  range  of  exercise  can  be  given  the  muscles  in  a  very  brief 
period,  provided  our  facilities  for  giving  them  are  sufficient.  A 
trial  case  and  trial  frames  will  answer,  if  there  are  no  better  means 
at  hand.  But  up-to-date  optometrists  who  give  these  exercises  find 
it  advantageous  to  have  special  facilities  for  giving  them.  A  few 
simple  devices  are  helpful  to  those  who  may  not  be  provided  with 
the  more  expensive  means  of  giving  these  exercises.  They  can 
obtain  these  later,  when  their  practice  in  this  field  has  paid  for  a 
better  equipment.  The  simpler  devices  referred  to  include,  among 
others,  the  following: 

Trial  Case  Devices 

Even  the  batteries  of  spherical  lenses  positive  and  negative,  may 
be  employed  for  muscular  exercise  upon  a  distant  target.  Plus 
sphericals  subdue  positive,  or  incite  negative,  accommodation ;  minus 


190  MUSCLES   OF  THE   EYE 

spherical  lenses  incite  positive,  or  subdue  negative,  accommodation  or 
ciliary  action  and  relaxation.  Alternation  of  imposing  and  remov- 
ing them  from  before  the  eyes  therefore  is  ciliary  exercise.  Placing 
the  pin-hole  disc  before  one  eye,  the  other  being  covered,  by  reduc- 
ing the  volume  of  light  incites  dilatation  of  the  pupil,  or  contraction 
of  the  radial  muscles  of  the  iris ;  while  its  removal  by  again  flooding 
the  eye  with  light,  causes  constriction  of  the  pupil,  or  contracts  the 
sphincter  muscles  of  the  iris,  and  relaxes  the  radials.  An  iris  dia- 
phragm, so  mounted  that  there  would  be  a  place  for  it  in  the  trial 
case,  and  it  could  be  inserted  in  a  cell  of  the  trial  frame,  would  im- 
prove facilities  for  giving  exercises  of  this  kind.  The  stenopaic 
slit,  by  rotations  from  90  to  180,  and  vice  versa,  alternately  exposes 
those  meridians  to  incident  light. 

The  battery  of  prisms  in  the  trial  case  provide  every  facility 
for  so  deviating  the  light  from  the  target,  just  before  it  enters  the 
eye,  as  to  put  an  immediate  tension  upon  the  muscle  under  the  apex 
of  the  prism  so  as  to  rotate  the  eye  and  give  its  visual  axis  the  direc- 
tion required  for  the  target  in  its  displaced  position,  or  apparently 
new  direction  from  the  eye  the  prism  covers.  But  the  other  eye,  un- 
covered, must  maintain  its  fixation  of  the  target,  and  therefore  be 
restrained  from  turning  with  the  eye  before  which  the  prism  is 
placed.  Hence,  a  pair  of  duction  muscles  are  incited  to  action,  what- 
ever may  be  the  position  of  the  prism  before  either  eye.  Removing 
the  prism  at  once  allows  such  muscular  action  to  subside,  and  this  is 
muscular  exercise  of  the  pair  or  pairs  of  duction  muscles  so  incited 
to  action.  This  may  not  be  the  handiest  way,  and  therefore  a  special 
mounting  of  a  series  of  prisms,  with  their  bases  all  in  the  same  direc- 
tion allows  successively  higher  powers  to  be  placed  before  the  eye. 
But  this  is  not  an  elegant  device,  and  rather  awkward  to  handle. 

Risley    Rotary   Prisms 

This  consists  of  a  pair  of  prisms  of  equal  power,  so  mounted 
in  a  metal  disc  as  to  permit  their  rotation  equally  in  opposite  direc- 
tions, and  with  a  mechanism  for  so  rotating  them,  having  a  handle 
that  projects  to  the  side,  which  is  operated  by  twisting  its  head  be- 
tween the  thumb  and  fingers.    The  handle  also  enables  the  operator 


MUSCLES   OF   THF.    EYE 


191 


to  place  it  in  any  desired  axial  position  in  a  trial  frame  cell,  and  to 
rotate  the  entire  device  without  disturbing  the  axial  position  in  the 
cell.  The  prisms  may  be  set  at  neutral  by  rotating  them  in  the  disc 
to  the  position  in  which  the  base  of  each  prism  is  opposite  the  apex 
of  the  other.  Then,  by  rotating  them  in  the  disc,  base  and  apex 
separate,  and  the  two  apices  and  two  bases  rotate  toward  each  other, 
finally  coming  together.  In  this  position  the  prismatic  power  is  that 
of  the  two  prisms  combined,  or  double  the  power  of  each.  But,  in 
their  rotation,  they  successively  add  to  the  power  of  one  direction, 
midway  between  the  axes,  or  in  the  direction  toward  which  their 
bases  rotate,  and  this  may  be  in  either  direction  by  opposite  turnings 
of  the  prisms. 


FIGURE  40. 

The  Risley  Rotary  Prisms:  A,  neutral  position,  for  rotation  to  horizontal 
power;  B,  horizontal  power,  base  in  or  out;  C,  neutral  position  for  rotation  to 
vertical  power;  D,  vertical  power,  base  up. 


192  MUSCLES   OF  THE   EYE 

If  a  pair  of  4 A  prisms  are  mounted  in  the  above  manner,  and 
the  prisms  are  set  at  the  neutral  position,  it  may  be  placed  with  the 
base-apex  lines  of  both  at  90  in  the  rotary  cell  of  the  trial  frame 
before  the  right  eye.  In  that  position,  rotation  of  the  prisms  in  the 
disc  at  once  develops  prismatic  power  at  right  angles  to  the  neutral 
position,  or  at  180.  In  one  direction  of  rotation  this  prism  power  is 
developed  base-in  before  the  right  eye,  in  the  other  direction  it  de- 
velops power,  base  out.  But  before  the  left  eye,  in  which  the  direc- 
tions of  the  base,  in  or  out,  are  opposite  to  those  for  the  right  eye, 
rotation  of  the  prisms  that  develop  prism  power  base  in  for  the  right 
eye  develops  prism  power  base  out  for  the  left  eye.  In  using  the 
Risley  Rotary  Prisms  for  the  purpose  of  developing  prism  power, 
one  needs  to  be  sure  of  the  effects  of  the  rotations  before  the  eye 
in  front  of  which  it  stands,  and  especially  if  he  uses  two  of  these 
devices  at  once,  one  before  each  eye. 

As  to  the  power  of  the  twin  prisms  in  a  Risley  Prism  device, 
the  stronger  the  prisms  the  more  rapidly  rotation  develops  their 
power.  A  pair  of  4A  prisms  in  such  a  mounting,  starting  from  neu- 
tral, develops  the  power  of  but  one  of  the  4A's  when  the  rotation 
carries  them  to  the  point  where  their  base-apex  lines  are  at  right 
angles  to  each  other,  and  it  is  on  the  line  midway  between  them. 
Rotation  of  90°,  or  from  vertical  to  horizontal,  develops  the  maxi- 
mum power  of  the  two  prisms,  the  sum  of  the  two,  but  along  the 
same  midway  line  between  the  two  base-apex  lines  of  the  prisms. 
The  development  of  prism  power  by  rotation  is,  from  the  neutral 
position,  at  first  very  slow,  for  it  is  relative  to  the  squares  of  the 
sines  of  the  angles ;  and  since  the  sines  are  fractional,  their  squares 
give  a  smaller  factor  than  the  sines.  The  sine  of  30°,  for  instance 
(sin  30°)  is  .5,  or  ^  of  a  radius;  but  its  square  is  but  .25,  and  this 
is  the  factor  of  value.  With  the  base  or  apex  of  a  prism  at  30°  from 
neutral  its  power  is '.25  of  that  of  the  prism  along  the  line  at  right 
angles  to  its  neutral  position.  Each  of  the  prism  values  follows  the 
same  rule.  But  these  values  are  usually  marked  off  on  the  Risley 
Prism  device,  and  for  the  prisms  it  carries. 

As  low  prism  values  may  be  obtained  by  slight  rotations  of 
strong  prisms,  the  twin  prisms  of  the  Risley  Prism  devices  are 
usually  quite  strong.     But,  for  exercise  purposes,  it  might  be  better 


MUSCLES  OF  THE  EYE  IM 

to  have  them  of  lower  power.  The  object  being  the  development  of 
control  by  exercise,  the  weaker  prisms  that  develop  sufficient  power 
and  not  too  rapidly,  ought  to  be  preferred  for  the  purpose.  The  full 
power  of  the  prisms  is  only  used  to  determine  the  maximum  duction 
power  of  the  muscles  after  a  period  of  exercise.  If  the  prisms  are 
used  in  pairs,  a  pair  of  the  Risley  Prism  devices,  one  before  each 
eye,  and  are  set  neutral  at  90  before  each  eye,  to  develop  prism 
power,  base  in  or  base  out,  they  are  rotated  in  opposite  directions. 
To  rotate  them  in  the  same  direction  develops  prism  power  base  in 
before  one  eye,  with  prism  power  base  out  before  the  other  eye,  or 
makes  the  prisms  neutral,  so  far  as  their  effects  in  producing  duc- 
tions  is  concerned.  They  will  produce  versions  rather  than  duc- 
tions,  for  the  bases  will  be  in  the  same  direction,  both  to  the  right 
or  both  to  the  left,  but  one  in  and  the  other  out.  The  objection  to 
a  single  device  is  that,  since  one  eye  only  rotates  back  of  the  prism, 
the  displacement  appears  to  be  in  that  direction  only,  and  to  the  eye 
before  which  the  prism  stands.  A  slight  turning  of  the  face  in  that 
direction  balances  the  duction,  however. 

There  seems  to  be  a  good  many  optometrists  who  are  stricken 
with  the  delusion  that  a  prism  before  the  right  eye  affects  the  mus- 
cles of  that  eye  only,  in  a  duction  test.  If  that  were  really  a  fact 
the  left  eye  might  be  covered  with  an  opaque  disc  while  testing  the 
duction  of  the  right  eye ;  or,  if  prism  power,  base  in,  were  developed 
before  the  right  eye,  prism  power  base  out  of  an  equal  amount 
simultaneously  developed  before  the  left  eye  would  not  affect  the 
right.  In  using  the  Risley  Prism  device  before  the  right  eye  to  de- 
termine the  adduction  power  of  the  two  eyes,  20°  or  20A  of  adduc- 
tion may  be  found.  If  the  same  test  is  made  with  the  device  be- 
fore the  left  eye,  it  will  not  be  essentially  different.  But  this  does 
not  mean  that  there  is  a  total  adduction  power  of  40 A,  but  only  of 
the  20 A,  as  found  and  verified. 

Rotary  Cylinders 

A  simple  method  of  exercising  the  oblique  muscles  is  to  employ 
two  equal  plus  or  minus  cylinders  and  insert  them  in  the  rotary  cells 
of  a  trial  frame,  one  before  each  eye,  with  their  axes,  to  begin  with, 
parallel,  as  both  at  90  or  both  at  180.    To  avoid  involving  the  ac- 


194  MUSCLES  OF  THE  EYE 

commodation  for  normal  eyes,  plus  cylinders  seem  best.  They  should 
be  of  sufficient  power  to  impair  vision  of  lines  under  the  axes  of  the 
cylinders,  as  either  -}-6  or  — 6  is  apt  to  do.  If  our  target  at  20  feet 
is  a  cross,  with  distinct  vertical  and  horizontal  lines,  a  pair  of  +6 
cylinders,  axes  180,  will  leave  only  the  vertical  lines  clearly  visible. 
If  the  two  cylinders  are  then  rotated  together  in  the  same  direction, 
they  will  tend  to  slant  the  clear  vertical  line  in  the  direction  the 
cylinders  are  rotated.  The  visual  sense  will  demand  such  action  of 
the  muscles  as  will  resist  this  tendency,  or  produce  a  corresponding 
version  of  the  eyes.  As  soon  as  the  vertical  line  assumes  that  appar- 
ent slant,  the  version  is  unable  to  overcome  the  displacement  and  the 
muscles  relax. 

Turning  the  cylinders  in  opposite  directions  will  tend  to  give 
the  vertical  line  a  double  appearance,  one  line  crossing  the  other.  To 
this  visual  anomaly  the  visual  sense  opposes,  or  incites  muscular 
opposition.  This  therefore  results  in  cyclo-duction  of  the  eyes,  en- 
gaging either  the  two  superior  obHque  muscles  together,  or  the  two 
inferior  obliques.  The  actual  appearance  of  the  crossing  of  the 
lines  indicates  that  the  effectiveness  of  muscular  resistance  to  the 
optical  displacement  of  the  lines  is  no  longer  adequate.  Practice  in 
this  exercise  will  soon  increase  the  efficiency  of  muscular  resistance 
to  the  doubling  of  the  vision,  and  represents  increased  control  of  the 
oblique  muscles  over  the  cyclo-ductions  of  the  eyes.  This  exercise 
has  been  found  beneficial,  particularly  in  cases  of  oblique  astigma- 
tism. Dr.  Savage  of  Louisville,  Ky.,  is  and  has  for  a  long  time  been 
its  leading  advocate.  His  book  upon  Ocular  Myology  gives  a  full 
account  of  his  theories  and  methods  of  giving  these  exercises. 

It  is  usually  found  that  oblique  astigmatism,  when  symmetrical, 
is  not  difficult  of  correction,  and  a  patient  with  this  sort  of  astig- 
matic obliquity  usually  accepts  the  cylindrical  correction,  and  wears 
it  with  comfort.  But  if  the  oblique  astigmatism  is  asymmetric,  it  is 
usually  quite  hard  for  the  patient  to  become  accustomed  to  wearing 
it.  In  all  cases  of  oblique  astigmatism,  whether  symmetric  or 
asymmetric,  cylindrical  exercises  such  as  here  outlined,  appear  to  be 
helpful.  They  tend  to  relieve  the  peculiar  qualities  of  asthenopia 
that  the  optical,  and  therefore  the  visual  defect  causes.  Primarily 
the  purpose  in  all  muscular  exercise  is  to  relieve  asthenopic  symp- 


MUSCLES  OF  THE   EYE  195 

toms,  muscular  asthenopia  it  is  called,  in  distinction  from  accom- 
modative asthenopia,  although  one  is  as  much  muscular  as  the  other. 
The  real  painfulness  of  vision  is  of  a  nervous,  rather  than  a  muscular 
character,  for  pain  of  any  variety  is  of  course  of  a  nervous 
character.  It  is  not  the  muscles  that  ache,  but  the  nerves  that  stimu- 
late them  that  are  in  distress. 

The  rotary  cylinders  have  been  elaborated  into  an  instrument 
that  corresponds  to  the  above.  This  must  not  be  confused  with  the 
rotary  cross  cylinders  that  are  used  for  measuring  or  verifying  the 
refraction  of  an  eye.  In  the  latter  the  double,  or  cross,  cylinders, 
rotate  a.xially  like  the  Risley  Rotary  Prisms,  both  being  before  one 
eye,  and  having  a  neutral  relative  position,  and  developing  cylin- 
drical power  by  their  rotations  in  relation  to  each  other.  In  the 
cylindrical  phorometer  a  pair  of  equal  cylinders  are  mounted  binocu- 
larly  for  rotation  in  the  manner  above  described.  Their  purpose  is 
to  give  exercise  to  the  oblique  muscles,  and  violations  of  fusion  of 
such  a  character  that  the  oblique  muscles  alone  are  able  to  overcome 
the  diplopia  produced. 

The  Amblyoscope 

This  instrument  is  a  modification  of  the  stereoscope  of  former 
days,  an  instrument  that,  with  the  advent  of  the  "movie"  has  become 
antiquated.  The  Amblyoscope  has  a  binocular  eye  piece,  and 
the  two  channels  for  the  admission  of  light  to  the  right  and  left  eyes 
separately  are  so  hinged  together  as  to  enable  the  operator  to  con- 
verge or  diverge  them.  There  is  a  prismatic  pair  of  lenses  before  the 
eyes  of  the  correct  power  to  neutralize  normal  accommodation  and 
convergence  for  the  distances.  But  there  is  a  separate  target  for  each 
eye,  the  same  as  in  the  stereoscope,  which  are  made  visible  by  direct 
or  trans-illumination.  The  targets  are  usually  two  pictures  of  sep- 
arate parts  of  an  entire  object,  such  as  a  bird  in  a  cage,  a  man  and 
a  horse,  a  person  and  his  hat,  a  boy  catching  a  ball,  etc.  There  is 
this  difference,  that  the  bird  is  not  in  the  cage,  the  man  is  not  on  the 
horse,  the  hat  is  not  on  the  man's  head  and  the  ball  is  not  in  the  boy's 
hands. 

By  the  adjustments  of  the  binocular  tubes  through  which  the 
light  from  these  targets  come  to  the  eyes  separately,  the  two  retinal 


196  MUSCLES   OF  THE   EYE 

images  are  brought  together.  At  the  right  point  of  adjustment  the 
bird  takes  his  position  in  the  cage,  the  man  is  on  the  back  of  the 
horse,  the  hat  takes  its  place  on  the  head  and  the  boy  catches  the 
ball.  That  is,  the  two  really  distinct  targets  are  "fused"  into  one, 
so  that  two  pictures  are  seen  as  one.  The  tubes  may  be  so  adjusted 
that  a  person  with  strabismus  and  binocular  diplopia  may  be  made 
to  see  singly.  When  a  child  is  in  this  condition,  as  often  happens, 
and  by  the  use  of  the  Amblyoscope,  the  bird  is  put  into  the  cage  or 
the  man  is  put  upon  the  horse's  back,  the  muscles  of  the  eyes  will 
resist  any  adjustment  that  tends  to  separate  the  two  pictures,  or  to 
break  up  fusion  of  the  images.  The  effect  of  muscular  exercise  of 
this  kind  is  to  so  develop  the  fusion  sense  that  the  muscles  of  the 
eyes  will  resist  separation,  and  diplopia  will  be  eliminated.  This  is 
the  first  step  in  restoring  the  eyes  to  normal  single  vision,  as  that  is 
essential  before  the  tendencies  of  the  eyes  to  deviate  abnormally  can 
be  attended  to,  or  the  muscular  imbalance  be  considered  and  treated, 
optically  or  surgically. 

If  the  Amblyoscope  possessed  no  other  utility  than  that  of  cul- 
tivating the  "fusion  sense"  in  strabismic  children,  it  would  still  be 
a  valuable  instrument  for  the  optometrist  to  possess.  It  affords 
means  for  exercising  the  muscles  that  are  involved  in  exercising  the 
ductions  required  for  the  fusion  of  the  images.  Under  its  manipu- 
lations any  person  whose  muscles  are  in  normal  balance  will  feel 
the  strain  that  it  puts  upon  the  muscles  to  maintain  fusion,  and  that 
is  of  course  muscular  exercise.  These  qualities  make  it  a  dynamic 
photometer  of  special  value,  although  it  is  not  advertised  as  such. 
It  is  an  ingenious  way  of  putting  the  principle  of  the  stereoscope  to 
practical  use  in  the  development  of  control  of  the  muscular  func- 
tions of  the  eyes.  The  writer  remembers  to  have  seen  a  crude  device 
of  this  kind  in  the  office  of  one  of  the  leading  optometrists  of  Indiana, 
now  on  the  state  board  of  optometry,  which  was  used  as  a  means  of 
determining  the  relative  influences  of  the  different  muscular  func- 
tions of  the  eyes  upon  each  other.  Though  simply  constructed,  it 
was  expected  that  the  device,  which  had  advantages  even  greater 
than  the  amblyoscope,  would  some  day  be  on  the  market  in  more  sub- 
stantial and  elegant  form.  But  that  was  ten  years  or  more  ago,  and 
it  has  not  yet  appeared.    There  are  a  good  many  valuable  ideas  that 


MUSCLES   OF  THE   EYE  197 

never  get  farther  than  this,  and  for  the  obvious  reason  that  they 
may  conflict  v^^ith  some  less  ingenious  device  already  being  manu- 
factured and  sold,  and  they  are  therefore  frowned  upon  by  the 
optical  trade,  with  whose  instruments  they  would  come  in  compe- 
tition. In  time  they  may  be  rediscovered  under  happier  circum- 
stances. 

The  Phorometer 

A  more  elaborate  instrument  for  testing  muscular  balance,  and 
giving  a  course  of  training  of  the  muscles,  one  that  embodies  all  of 
the  dififerent  means  we  have  described  under  this  subject,  is  the 
Phorometer.  It  may  be  described  as  an  elaborated  trial  frame,  for 
it  provides  all  of  its  facilities  in  adjusting  it  to  dififerent  patients,  that 
a  trial-frame  has,  and  many  more.  It  is  in  fact  a  phoro-optometer. 
It  is  usually  mounted  upon  a  stand  or  bracket,  and  may  be  swung 
into  position  pivotally,  and  then  adjusted  in  height,  level  and  pupil- 
lary width,  bridge,  etc.,  to  the  patient.  As  side  parts  there  are  one 
or  two  Maddox  rod  discs  that  are  swung  into  position  from  the  side. 
A  single  or  two  of  the  Risley  Prism  devices  are  similarly  available. 
The  trial  case  lenses  may  be  inserted  in  special  cells  provided  for 
them,  or  be  mounted  in  circular  discs  that  rotate  them  before  the 
eyes,  thus  providing  any  lens  value  that  may  be  required  for  either 
or  both  eyes. 

Those  who  have  one  of  these  instruments  complete  make  it  serve 
all  of  these  purposes,  both  that  of  the  trial-frame  and  the  phorom- 
eter. They  are  usually  finished  in  black  enamel  or  lacquer,  or  in 
a  nickel  finish  for  some  of  the  adjustable  parts.  These  give  the  in- 
strument an  elegant  appearance,  which  has  much  to  do  with  their 
impression  upon  the  patient.  There  are  several  makes  or  designs 
of  an  instrument  of  this  kind,  each  having  special  advantages  and 
facilities  that  cannot  be  individually  discussed.  It  is  for  the  buyer 
of  one  of  these  instruments  to  make  his  own  selection,  weighing  tlieir 
respective  merits  as  they  are  demonstrated  to  him  by  a  sales  repre- 
sentative. Optometry  has  been  materially  advanced  by  the  class  of 
instruments  furnished  by  the  manufacturers  of  these  and  other  in- 
struments, and  one  may  consider  that  he  is  contributing  to  the  gen- 
eral advancement  by  having  and  operating  one  of  them.    They  may 


191  MUSCLES  OF  THE  EYE 

be  bought  in  different  degrees  of  elaboration,  like  an  automobile,  so 
that  one  may  decide  the  matter  for  himself  as  to  the  fullness  of  the 
equipment  and  the  amount  of  his  investment  in  it. 

Genothalmic  Refractor 

This  newest  instrument,  though  named  as  above,  is  really  an 
Opto-Phorometer.  That  is,  while  embodying  facilities  for  measur- 
ing the  refraction  of  the  eyes,  it  also  has  the  facilities  for  measuring 
and  exercising  the  muscles.  It  is  suspended  before  the  eyes,  which 
facilitates  adjustments.  The  cylindricals  are  also  nearest  the  eyes, 
the  spheres  in  front.  It  is  the  "final  word"  in  instruments  of  this 
kind,  and  the  optical  company  who  are  putting  it  on  the  market 
warrant  that  it  will  be  as  near  perfection  for  its  purposes  as  it  can 
be  made. 

DeZeng  Phoro-Optometer 

This  was  the  first  instrument  of  its  class  to  appear  and  it  has 
always  held  a  prominent  place  in  the  equipment  of  the  up-to-date 
practitioner.  The  facilities  of  this  instrument  have  already  been 
discussed  in  detail,  therefore  do  not  require  repetition  here.  Suffice 
to  state  that  long  experience  of  the  manufacturers  in  developing  it 
has  made  it  an  instrument  of  precision. 

The  Woolf  Ski-Optometer 

The  significance  of  the  prefix  in  this  title  is  the  use  of  it  for 
shadow  testing  the  eye,  or  for  putting  the  required  lenses  before  the 
eyes  in  shadow  testing.  It  has  a  special  system  for  changing  the 
values  of  the  lenses,  and  also  for  placing  cylinders  in  any  axis,  before 
the  eye  being  tested.  In  other  respects  it  is  similar  to  other  phorom- 
eters.  One  needs  to  see  it  demonstrated  to  appreciate  all  of  its 
superior  features. 

General  Remarks 

What  may  be  done  to  relieve  the  eyes,  or  to  relieve  suffering 
humanity  of  eye-strain,  as  the  primary  cause  of  many  organic  dis- 
turbances and  symptoms,  by  muscular  exercise  through  the  employ- 


MUSCLES   OF  THE   EYE  199 

mcnt  of  lenses,  prisms  and  natural  means  of  inciting  the  muscles  to 
action,  is  still  a  prospect  rather  than  an  accomplishment. 

There  are,  of  course,  many  different  ways  of  trying  out  the  eyes 
to  determine  their  true  muscular  condition.  We  have  only  given  a 
few  of  the  more  important  ones;  for  the  others  embody  the  same 
principles  and  are  used  in  the  same  way.  The  optometrist  may 
choose  between  the  different  devices  and  methods,  using  that  method 
that  he  seems  to  obtain  the  best  results  from.  There  are  now  many 
who  are  working  out  systems  for  this  purpose.  Their  experiences 
appear  from  time  to  time  so  that  one  must  keep  in  touch  with  cur- 
rent literature  on  the  subject  to  be  really  up  to  date.  But  first  of  all 
he  must  be  grounded  in  the  fundamental  principles,  so  as  to  be  able 
to  read  the  signs  intelligently,  and  to  make  his  remedy  apply  to  the 
case  in  hand. 


— -  -  ^^^  .     .  ■     I  .    ■  , ,  ,  ,,.. 


Oculo-Refractive  Cyclopedia  and  Dictionary — ■ 

By  Thomas  G.  Atkinson,  M.  D,  Complete,  con- 
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FOLDERS 
Interesting  Facts   Concerning    Your  Eyes — 

One  of  the  most  popular  folders  ever  used  by  the 
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New  Wrinkles— Another  fine  folder  widely  used. 
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Published   by 

The  Professional  Press,  Inc. 

17  No.  Wabash  Ave.  CHICAGO 


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